求英语大师解

求英语大师解
1.Our math teacher is very _________(experience)
2.家长应该对十来岁的孩子严格要求.
Parents should ______ _______ ________ teenagers.(翻译）
3.I can't choose which pair of jeans to buy.（改同义句）
I can't choose which pair of jeans ______ ______ buy.
4.Li Li and Na Na didn't go to school today.I don't know what________ to them.( )
A.had happend B.were happend C.happend D.happens
表明原因
happend 没有被动语态