答案解释：
D【解析】这道题问第三段里的one指代的是什么。由第三段中“The president has started using?look-alikes during some public appearances．Dave is?offered a chance to‘serve his country’by becoming?one．”可知在公共场合露面时，总统开始使用替身，Dave有了一次做替身为国效力的机会。One?用来代替前面出现过的名词look-alikes的单数形式，也就是D项“a look．alike”。
