摘要：Many people say they have seen Unidentified Flying Objects, which they believe alien spaceships. A.are B.is C.have D.has
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Many people say they have seen Unidentified Flying Objects, which, they believe, ________alien
spaceships.
spaceships.
[ ]
A. are
B. is
C. have
D. has
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B. is
C. have
D. has