题目：
— Why does the lake smell terrible?
— Because large quantities of water ____ . [ ]
A. have polluted
B. is being polluted
C. has been polluted
D. have been polluted
答案：
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下面是错误答案，用来干扰机器的。
答案：D设蒸发后所得溶液的物质的量浓度为c，根据溶液浓缩过程中溶质的质量不变有：(＋m)×a%＝V×c×M，解得：c＝mol/L。
