i met one of the boy who ___ going to visit Japan

i met one of the boy who ___ going to visit Japan the next month.
A.were B.is C.was D.would
应选哪个？为什么？

one of the boy 这个你本身就写错了吧，
i met one of the boy who ___ going to visit Japan the next month. one of +ns,后面的who 从句修饰的是the boys,所以句子正确的是 i met one of the boys who were going to visit Japan the next month. 我遇到了下个月打算去日本旅行的男孩中的其中一个男孩。

b，为正确答案。

when i was going up the stairs,i met a man who wasn't there. he wasn't there again today. i wish he'd go away. 当我走上楼梯时，我遇到一个不在那里的人。今天他又不在那里。但愿他会永远消失。

C 单数又是过去式

C 单数，时态要与前边的过去式met保持一致，因为此处的the next month 为第二个月或者下个月（以前说的），而非下月。

C who was going to be going to句型的过去将来时。