The book was written in 1946, ____ the education system has witnessed great changes.
#70115
A
when
B
during which
C
since then
D
since when
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_____several times,but unfortunately,the boy still couldn’t smooth away the problem on his own.
#73673
A
Having been explained
B
The problem having been explained
C
The problem had been explained
D
Although the problem had been explained
—Could you do me a favor?—It depends on _______ it is. 
#72185
A
which
B
whichever
C
what
D
whatever