遗传是指亲子间的相似性,变异是指亲子间和子代个体之间的差异。 [ ]
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— Look! The black clouds are gathering, which indicates that it __________ rain soon.
— Yes, black clouds are the _________ of the rain. [ ]
A. is going to; signal
B. will; mark
C. is gong to; sign
D. is to; sign
平均单井产量是油田实际产量除以油田油井的总数。