英语主谓一致问题Hard struggle and plain living _ a weapon with which

英语主谓一致问题
Hard struggle and plain living _ a weapon with which our army and people overcame the enemy.
A.is B.are
I've told you that the third and the last part of the story _ most interesting.
A.is B.are
The culture and customs of America _ more like _ of England than of any other country.
A.is that B.are that
A A B
请简析原因