利用室内外空气流动造成的和空气温度差造成的，使空气流动的通风系统称为自然通风。
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按照作用范围的不同，通风系统可以分为通风和通风系统两大类。
I have a lot of books, half of ________ novels.
A. that
B. which
C. them
D. whom
The girl in the snapshot was smiling sweetly, her long hair _________.
A. flowed in the breeze
B. was flowing in the breeze
C. flowing in the breeze
D. were flowing in the breeze
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