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请解释什么是氢键，并说明氢键的两个特点。

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请解释什么是分子结构。

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