答案是A
记住以下句子,对你的高考有用：
1 He is only one of the students that / who are from London.(注意：only的前面没有the)
这里are的主语是the students,不是only one.
2 He is the only one of the students that is from London.(注意：only的前面有the)
这里第二个is的主语是the only one,不是the students.