It is ten months since he a doctor. A. becomesB. becameC. has becomeD. had become

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A. bees
(答案→)B. became
C. has bee
D. had bee
解析：考查时态。本题是一个固定句式。Since自从…；后面的句子使用一般过去时，前面的主句使用现在完成时或者一般现在时；本题我们的it is可以换成it has been…；句意：自从他成为一名医生到现在已经10年了。说明他是10年前做医生的。故B正确。