He was directly invited to join their party, but he declined it,
observing that he could imagine but two motives for their choosing to walk up
and down the room together, with either of which motives his joining them would
interfere.
两位小姐立刻请他来溜达，可是他谢绝了，说是她们俩所以要在屋子里踱来踱去，据他的想象，无非有两个动机，如果他参加她们一起散步，对于她们的任何一个动机都会有妨碍。
请教曹老师，这里的which是限定词还是定语从句的用法？
