答案：A

解析　在非限制性定语从句中，当从句置于句首时，或从句意为“正如……”时，只能用as而不能用which来引导从句。C项和D项中第一空若要用it，则应把前半句改为“It was expected that only five people turned up at the party”此时it作形式主语。第二空是which引导的非限制性定语从句，且在从句中作主语。