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【单选题】
滤层过滤过程中,有时在某一深度处会出现负水头现象,其主要原因是( )。A.
滤层截留杂质后，空隙中流速变大，携带入大量空气；
B.
滤层截留杂质后，过滤水量变小，溶解空气析出；
C.
滤层截留杂质后，某一深度水头损失超过该处水深；
D.
滤层砂面上水深偏大，过滤水头偏高。
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