for his good skill this doctor has a lot of visitors calling for help all the time.A. Kno

问题补充：

for his good skill,this doctor has a lot of visitors calling for help all the time.

A. Knowing

B. To know

C. Known

D. To have known

答案：

A. Knowing

B. To know

(答案→)C. Known

D. To have known

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