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

题型：不详难度：来源：

He found it increasingly difficult to read, ______ his eyesight was beginning to fail.
A. though         B. for        C. but          D so

答案

B

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。