With the help of high technology, more and more new substances ___in the past few years.
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A
discovered
B
have discovered
C
had been discovered
D
have been discovered
推荐题目
David _____ a customer when the fire started．
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A
served
B
was serving
C
had served
D
would serve
Don’t disturb me. I_______ letters all morning and have written ten so far.
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A
have written
B
am writing
C
was writing
D
have been writing
Would you please keep silent? The weather report and _________I want to listen.
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A
is broadcast
B
is being broadcast
C
has been broadcast
D
had been broadcast
Listen to the two girls by the window. What language _________?
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A
did they speak
B
were they speaking
C
are they speaking
D
have they been speaking
So far nothing _____ Tom. I want to know the reason _______ he hasn’t written to me.
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A
heard from; why
B
has been heard from; why
C
was heard from; that
D
has heard from; that