题目内容

假设某医生每天门诊挂号的病人数为N,是服从参数为a的泊松分布随机变量。又假设每个病人门诊看病的时间也为随机的，均服从参数为b的指数分布随机变量，且相互独立。这名医生总的门诊看病时间为T,则E(T)为（）

A. E(X1)
B. E(N)
C. E(X1)+E(N)
D. E(X1)E(N)

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