The visitors lost their way in the forest ; made things worse. was that it began to rain.A

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A. that
B. which
C. who
(答案→)D. what
解析：what引导主语从句，作主语从句的主语；that引导主语从句，不做成分，本题中was前面是主语从句，后面是that引导的表语从句。句意：旅行者们在森林中迷路了，更糟糕的是天气还开始下雨了。