Trial and error are the source of our knowledge.A、isB、wereC、hasD、have beenA时

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问题 Trial and error are the source of our knowledge.

选项 A、is
B、were
C、has
D、have been

答案 A

解析时态应用。短语trial and error(反复实验)是单数主语，后面谓语动词应用第三人称单数的"is"而非"are"。

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