What the doctors really doubt is my mother will recover from the serious disease soon. A.

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A. when
B. how
(答案→)C. whether
D. why
解析：主要测试表语从句。此类试题的解决方法主要应该从所给答案以及句子     my mother will recover from the serious disease soon来分析可知选择whether最佳。译文：医生真正怀疑的是我母亲是否能够从严重的疾病中快点恢复。