根据软硬酸碱原理，请说明硬酸、软酸、硬碱和软碱在结构上各有什么特点。
There _______ many apples on the tree last year.
A. have been
B. were
C. are
D. is
How many _____ are there in the room?
A. apple
B. students
C. milk
D. paper
He said that they _______ Yunnan.
A. Have been to
B. Had gone to
C. Have gone to
D. Has gone to
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