One of the most important questions they had to consider was of public health. A. whatB.

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A. what
B. this

(答案→)C. that
D. which
解析：考查代词。分析句子结构可看出they had to consider是定语从句，可见was是系动词，根据选项可判断后面不是句子，不需要疑问词，排除A和D。通过句子结构的分析空格应是指上文的question这个名词。语法告诉我们that指上文，而this指下文。句意是“他们不得不考虑的最重要的问题之一就是公众健康的问题”。