He found it increasingly difficult to read his eyesight was beginning to fail.A. though

问题补充：

He found it increasingly difficult to read, his eyesight was beginning to fail.

A. though B. for C. but D so

答案：

解析：这是一个原因连词。原因连词for表示的原因并不说明主句行为发生的直接原因，只是提供一些有利于说明情况的补充说明, 且不可位于主句前。