几道高中英语语法题目，求详细解答

Youcannotacceptanopinion_____toyouunlessitisbasedonfacts.

A.offering B.tooffer C.havingofferdD.offerd 都是被动，为何选C而不选D？

Theywaitedpatientlyfor___seemedaverylongtime,stampinginthesnowtokeeptheirfeetwarm.

A.which B.it C.when D.what 为什么选D？

Withmanymajoreconomicproblems_____(remain)tobeaddressed,thereisnopoint____(predict)thegrowthrateofthisyearnow.

答案分别为remaining;predicting.为什么不是remained;topredict

---HaveyoubeentotheUS?

---Yes,onlyonce.I____thereonlyforsevendays.

答案是stayed.为何不是hadstayed或havestayed?

Whatwouldhavehappend_____thathasjustbeenreportedashavingcrashed?(答案A，完全不懂)

A.Ihadtakentheflight B.ifIshouldtaketheflight C.hadItakentheflight D.ifItooktheflight

urge后接宾语从句，也要用should+动词原形，should可以省略吗？

Ijustdon'tunderstand____thatpreventssomanyAmericansfrombeingashappyasonemightexpect. 空中填whatitis.为什么不是whatitdoes?

Dr.Bethuneworkedhardasifhe_____.答案选B为什么

A.neverhadfelttired B.hadneverfelttired C.neverfelttired D.wastirednever