Could someone explain why B is wrong for Q20? Thanks! #help

This one is not that hard. I found that since they are looking for the same mistake that both arguments made. I just read the first one and found it's a reverse necessity with sufficient kind of mistake. Then I stop reading. I think in that way we can save a lot of time.

This English! Is killing me.

Why Q4 is a necessary assumption but not a sufficient assumption?

#help (Added by Admin)

I am interested