【题目】He sometimes asked himself the question ______ his research was worth devoting his life to.
A．that B．if C．whether D．how
在线课程【答案】C
【解析】句意：他有时候问自己这个问题——他的研究是否值得将他的毕生都投入进去。根据句意，可见是表示怀疑，所以要选表“是否”的词；而空缺处引导同位语从句，______ his research was worth devoting his life to作the question的同位语，if不能引导同位语从句，故选C项。