The old lady's hand shook frequently. She explained to her doctor ________ this shaking had begun half a year before, and ________, only because of this, she had been forced to give up her job.

A.when; how

B.how; when

C.how; how

D.why; why

答案及解析： C

解析：

两个空均需填入连接词引导的名词性从句，是explain的两个宾语从句。when表示时间，而从句中有时间状语half a year before。why表示原因，从句中也出现了原因状语only because of this。因此选择how，表示方式，“怎么样”。