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

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：

A. though

B. for

C. but

D. so

解析：B

解析句意为：他发现读书越来越困难，因为他的视力开始下降。for表示原因，符合句意。