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题目内容
（河北农业大学大学英语Ⅳ）
The scientists of the research center keep the good habit of summing up their experience _______ .
A、
at one time
B、
at a time
C、
from time to time
D、
at no time
A、
at one time
B、
at a time
C、
from time to time
D、
at no time
参考答案
