Though ________, swimming does us much good．
A.tiredB.we tire outC.tiringD.it is tired在线课程C
解析：
分析句子的结构和逻辑关系，此句前后是让步关系。可以把though remember 看成是一个省略句，相当于：Though swimming is tiring，这里swimming与tiring是系表结构，表示属性，所以选tiring。