It __be the vocabulary that caused you the problem in the exercise because you know a lot of words.

When should I hand in mypaper?— Your paper must _as soon as the bell_ .

_____ he can pass the driving test is quite uncertain.

Can you play chess— No, but I can play ________ piano very well．

He is rather difficult to make friends with, but his friendship, ________, is more true than any other.