I found this book on my desk when I came ...

★ Passive Modals

I found this book on my desk when I came to class. It — (leave) by one of the students in the earlier class.

A. must have left

B. must be left

C. must have been left

D. must have be left

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Your daughter has a good voice. Her interest in singing —- (encourage).

A. must encourage

B. must be encouraged

C. encourage

D. must be encourage