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

(答案→)B. for

C. but

D. so

解析：句意为：他发现阅读逐渐地困难起来，因为他的视力开始慢慢下降。A项意为“尽管”，B项为“因为，由于”，表明附加或推断的理由，引导的分句前常有逗号，for分句一般不放于句首。But表示转折，so表示“因此”。根据句意可以知道，后面一句话是用来解释说明前一句的，所以用B项最佳。