【正确答案】 D

【答案解析】[解析] though可以引导让步状语从句，例如：I think the poem is very good, though it's written by an obscure poet. 我认为这首诗很好，尽管它出自一位默默无闻的诗人之手。who可以引导定语从句，例如：This man was beloved by all who knew him. 这个人为所有认识他的人所敬爱。if可用来引导条件状语从句。while可以引导时间状语从句，例如：He asked me to hold on while he left the telephone to find a pencil. 他让我等一等别挂电话，他去找支铅笔。