---Mary hasn't written to us for a long time.---What do you suppose __ to her?
#73595
A
happened
B
to happen
C
was happened
D
having happened
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I have two grammars, ______are of great use. 
#73806
A
all of which
B
either of which
C
both of that
D
both of which
__________for the bus, I heard a noise,
#69550
A
While waiting
B
While I waiting
C
I was waiting
D
Waited