Scientists say it may be five or six years ______  it is possible to test this medicine on human patients.
#92459
A
before
B
after
C
since
D
when
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Rather than _________ on a crowded bus, he always prefers _________ a bicycle.
#92690
A
ride; ride
B
riding; ride
C
ride; to ride
D
to ride; riding
It was a serious mistake, ______ caused by carelessness. 
#93814
A
which I think was
B
which I think it was
C
I think which was
D
that I think it was
Not until the car looked almost new____ cleaning it.1网
#93516
A
John stopped
B
did John stop
C
stopped John
D
John did stop