Dr. Ma used to two or three persons in the hospital.
A. operate on
B. operating on
C. operate for
D. operating for
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(答案→)A. operate on
B. operating on
C. operate for
D. operating for
解析：operate on对……做手术； operate for运营。句意：在医院里，马大夫过去常常给两到三个人做手术。used to do表示过去常常做某事，to是不定式符号，后面接原形动词，结合语境可知选A。