He found it increasingly difficult to read, ______ his eyesight was beginning to fail.
A.thoughB.forC.butD.so在线课程B
解析：
根据句意可知，前后两句话之间是因果关系，并且此处表示一种补充说明的原因，前面有逗号，应该用for。其他三项不能表示因果关系。故选B。