请问,after the fire,___would otherwise be a cultural center is

请问,after the fire,___would otherwise be a cultural center is now reduced to a pile of ashes.
after the fire,___would otherwise be a cultural center is now reduced to a pile of ashes.
选项a.that b.it c.which d.what
因为是非限定性定语从句所以不能选a.b.好像也不太合适,但c为什么不可以呢?
请问，which引导的会是什么性的从句呢？