One of the most important questions they had to consider was of public health.
A. what
B. this
C. that
D. which
网友回答
A. what
B. this
(答案→)C. that
D. which
解析：考查代词辨析。That指代可数名词单数或者不可数名词，后面必须要有定语修饰。本题的that指代the problem，后面有定语修饰。句意：他们不得不考虑的最重要的问题之一就是公众的健康问题。故C正确。