A:when the earthquake took place in Wenchua
B:when the earthquake has taken place in Wenchua
C:when did the earthquake take place in Wenchua
D:when the earthquake will take place in Wenchua
某校生物课外兴趣小组进行人工孵化蛙卵的实验,实验记录如下表所示(容器大小、孵化时间等均相同),根据下表推断,该实验的实验变量是:( )
组别
水源
水量(mL)
水温(oC)
蛙卵数(个)
孵出蝌蚪数(个)
①
池塘水
500
24
250
245
②
13
165
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