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Thomas Edison ____ already ____ a chemistry lab for himself

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Thomas Edison ____ already ____ a chemistry lab for himself by the time he ____ ten.
A.has…built…was B.had…built…was C.would…build…was D.was…building…is
B,因为从句是一个过去时,所以主语是发生在过去的过去,所以用过去完成时.