By the time Neil Armstrong and Edwin Aldrin walked on the moon in 1969, there ______ many space explorations.
A.will have been B.had been C.have been D.must be正确答案B
By the time Neil Armstrong and Edwin Aldrin walked on the moon in 1969, there ______ many space explorations.
A.will have been B.had been C.have been D.must be正确答案B