A question (and answers) to think about – why did Noach survive the flood???

In hilchos teshuva (3;2) the Rambam writes that if the world reach a status of their sins outweighing their merits they are ALL destroyed immediately, and brings a proof from the flood. The question is if all are meant to be destroyed, why did HaShem spare Noach and his family?
A1) The destruction the Rambam is talking about is the spiritual judgment in Olam HaBa, not physical death and destruction in this world (Lechem mishna there; the kesef mishna argues)
A2) The rule of the Rambam that the tzaddik is destroyed along with everyone else is only a tzaddik that has more miztvos than sins (eg 51-49% mitzos, 70-30% etc). But Noach was a 100% tzaddik, with no sins, and so the rule does not apply to him or his family. (Brisker Rav quoted by R’ Ezriel of Gateshead)
A3)Noach ordinarily should not have survived under the normal rules, but HaShem did him a special chesed to keep him alive; ‘And Noach found favour in the eyes of HaShem’ (6;8) (R’ Ezriel).