问题补充：
Although he failed when he tried doing the experiment for the first time, he decided to try it.
A. once in a while
B. at once
C. once more
D. once
答案：
A. once in a while
B. at once
(答案→)C. once more
D. once
解析：本题考查副词短语。句意：虽然他第一次做实验时失败了，可他决定再试一次。once more 再一次, 又一次。
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