题目内容：
第49题____________
A．A limited water supply. B．so that it can be used again just as if it were fresh from a spring.
C．It is possible to purify large amounts of sea water.
D．But even if every large city purified and reused its water.
E．And we cannot even use all of that.
F．Experiments have already been done in this field.
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答案解析：