问题
选择题
The students put up some signs ________ the riverside ________ the water clean. [ ]
A. by, to keep
B. at, to keep
C. by, keep
D. in, for
答案
答案：A
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The students put up some signs ________ the riverside ________ the water clean. [ ]
A. by, to keep
B. at, to keep
C. by, keep
D. in, for
答案：A