单选题 They went on well at first but eventually everything ______ they had worried about happened.

【正确答案】 C
【答案解析】[答案精解] 此题考查定语从句。因worried about后要接宾语，所以此处使用关系代词引导定语从句，由此可以排除D；what不可以引导定语从句，所以排除B；当先行词为不定代词everything时，只能使用that引导定语从句。