____ most bamboo blooms every year, there are some species that flower only two or three times a century.
A. Whenever
B. That
C. While
D. However
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_____some people criticize graffiti for being ugly and destructive, those graffiti lovers still see it as a real art.
As
B. While
C. Once
D. Until
—What happened to Tom?—He was crossing the street_____a motorbike hit him from behind.
A. while
B. when
C. until
D. because
I was halfway back to the cottage where my mother lived ______ Susan caught up with me.
A. when
B. while
C. until
D. through
We had just had our breakfast ______ an old man came to the door.
A. when
B. while
C. until
D. through