Answer #3: 'One may choose to follow...' teaches about something else, i.e. if two Tana'im or two Amora'im argue like Beis Shamai and Beis Hillel (i.e. before the Halachah was fixed like one opinion), one should not follow the leniencies of both, nor the stringencies of both. Rather, one should consistently follow one of them.

Recap of Question: The people of Neharda'a did improperly to follow the stringencies of Rav and Shmuel [regarding the bent Mavoy]!

Answer #1 (Rav Nachman bar Yitzchak): They followed Rav totally:

(Rav Huna citing Rav): The Halachah follows the first Tana, but we do not publicly teach this.

Question: Rav Ada bar Ahavah cited Rav to say that the Halachah follows the first Tana, and we publicly teach this. How can he answer?

Answer #2 (Rav Shizbi): One should not follow stringencies of different Chachamim only if they are contradictory, like the law of a spine and skull:

(Mishnah): An incomplete spine or skull [of a Mes does not have Tum'as Ohel];

Question: How much must be missing from the spine [to be considered incomplete]?

Answer #1 (Beis Shamai): The Shi'ur is two vertebrae;

Answer #2 (Beis Hillel): The Shi'ur is one vertebra.

(Beis Shamai): If the width of a drill-screw is missing from the skull [it is not Tamei b'Ohel];

Answer #2 (Beis Hillel): The Shi'ur is the amount which if removed from a live person would kill him.

(Rav Yehudah): The same amount missing makes an animal Tereifah. (It is foolish to be stringent both ways, to say that the smaller Shi'ur of Chisaron makes a Tereifah, but there is Tum'as Ohel unless the larger Shi'ur is missing.)

When the stringencies do not contradict each other, one may follow both.

Question (Rav Mesharshiya): Even when they contradict each other, one may follow both stringencies!

(Beraisa): R. Akiva once picked an Esrog (Rashi - tree) on the first of Shevat and separated one tithe like Beis Shamai [who say that Rosh Chodesh Shevat begins the new year for trees. This began the third year of the Shemitah cycle, so he gave Ma'aser Oni] and Ma'aser Sheni like Beis Hillel [who hold that it was still the second year, for the new year begins on Shevat 15].

Answer: No, R. Akiva was unsure whether Beis Hillel hold that the new year is the first or 15th of Shevat, therefore he was stringent. (Ran - when in doubt about a mid'Oraisa law, one must be stringent like both opinions.)

(Rav Yosef citing Rav Yehudah citing Rav): Chachamim and Chananyah argue about how to be Me'arev a Mavoy Mefulash that is open to Reshus ha'Rabim on both sides. However, if one side opens to a valley (Karmelis) and the other to Reshus ha'Rabim, or both open to a valley, it suffices to make Tzuras ha'Pesach on one side and Lechi or Korah on the other.

Question: Since he taught what permits when one side opens to Reshus ha'Rabim, there is no need to teach about when both sides open to a valley!

Answer: He means that if one side opens to a valley and the other to Reshus ha'Rabim, it is as if both open to a valley, [so it suffices...]

(Rav Yosef citing Rav Yehudah): If one side [of a Mavoy Mefulash] opens to a Rechavah (a Chatzer in back of houses, and the wall between it and Reshus ha'Rabim was breached), no Tikun is needed [on that side of the Mavoy].

Question (Abaye): The last law you said in the name of Rav Yehudah is like Shmuel. It is not like Rav for two reasons!

(R. Yirmeyah bar Aba citing Rav): If [the middle wall of] a Mavoy was totally breached [and is now open] to a Chatzer, and the Chatzer was also breached [on the opposite side, less than 10 Amos, and the ends of the Chatzer walls remain], the Chatzer is permitted (one may carry inside it), but the Mavoy is forbidden.

Reason #1: (The Mavoy is forbidden only because also the opposite wall of the Chatzer was also breached. It is like a Mavoy Mefulash.) Rav Yehudah would permit this, just like when it is open to a Rechavah! (Tosfos - because one can turn to the sides after leaving the Mavoy, we are not concerned lest it be confused with a Mavoy Mefulash.)

(Reason #2 - Rashi - surely, Rav Yehudah taught about a Mavoy open to a Rechavah because people do not dwell there. However, if it were open to a Chatzer, it would be forbidden even if it was not Mefulash! Tosfos - if not for the breach, we would not be concerned for being open to the Chatzer, because an [intact] end of the Chatzer wall is Nir'eh miba'Chutz v'Shavah mibi'Fnim (noticeable from outside the Mavoy, but not from the inside). It is considered a Lechi. This is unlike Rav Yehudah, who permits only a Rechavah, but forbids a Chatzer!)

Rav Yosef: I do not know from whom Rav Yehudah learned his law. I just know about a case in a village of shepherds:

A Mavoy [Mefulash] opened to a Rechavah on one side, and Rav Yehudah did not require any Tikun!

If it is difficult to say that he heard this from Rav, we can say that he heard it from Shmuel! (He was a Talmid of both of them.)

Answer: According to Rav Sheshes' explanation of Rav, we can say that Rav Yehudah learned this (the Mavoy open to a Rechavah) from Rav!

(Rav Sheshes): Rav forbids a Mavoy totally open to a Chatzer only due to the residents of the Chatzer, when they did not make an Eruv with the Mavoy. If they did, the Mavoy is permitted;

Rav permits a Mavoy open to a Rechavah, for [it is as if] there is an Eruv (there are no residents to forbid). He forbids [when it is open to a Chatzer, which has residents] when there is no Eruv.