单选题 A nation's merchant marine is made up of its commercial ships and people ______ them.
A. they operate B. who operate
C. they operate of D. do they operate

【正确答案】 B
【答案解析】该题是语法结构题，考查定语从句的用法。该定语从句的先行名词是people，在定语从句中做主语。其他选项不能这样用。因此，根据题意和语法规则，选项B为最佳答案。