One of the most important questions they had to consider was of public health.
A. what
B. this
C. which
D. that
网友回答
A. what
B. this
C. which
(答案→)D. that
解析：考察代词。本句中的系动词was后面不是一个完整的句子，所以不能使用AC项。That指代前面的名词problem，后面有定语修饰，表示特指。句意：他们不得不快乐的问题之一就是公共健康的问题。故D正确。