It was only then I began to have doubts about whether my story had ever been told.A. when

问题补充：

It was only then I began to have doubts about whether my story had ever been told.

A. when

B. Since

C. that

D. yet

答案：

A. when

B. Since

(答案→)C. that

D. yet

解析：句意：只有在那时我才开始怀疑是否我的故事被讲过了。这里是强调句，结构是：It is/was+强调部分+that…，判断是否是强调句的方法是去掉强调结构，这句话仍然成立，就是强调句，这里如果去掉强调结构是I began to have doubts about whether my story had ever been told then.所以是强调句，选C。