Climbing mountains was ________, so we all felt ________.
A.tiring; tiredB.tired; tiringC.tiring; tiringD.tired; tired在线课程A
解析：
动词加ing或ed转化成形容词，形容词时ing形式与ed形式区别在于：tiring表明“爬山”的本身是一件累人的事情，tired指人的感受。