单选题
The step-test was given ______.
A、
at the beginning and at the end of the 7-week period
B、
after each exercise period
C、
only at the beginning of the 7-week period
D、
only at the end of the 7-week period
【正确答案】
A
【答案解析】
解析：由文中可知，在为期7周的实验始末分别进行了一次阶梯测试。
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