6. Darwin proved that natural selection ___ the chief factor in the development ofspecies.
A) has been
B) had been
C) is
D) was
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7. While Peggy ___, her brother is playing records.
A) reads
B) is reading
C) has read
D) has been reading
8. It’s been a long time since I ___. How are you?
A) had last seen you
B) saw you last
C) have least seen you
D) last was seeing you
9. We ___ on it for several hours but we have not yet reached any conclusion.
A) work
B) are working
C) have been working
D) have been worked
10. He said that he ___ for Shanghai the next day.
A) will leave
B) has left
C) would leave
D) had left
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