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

问题补充：

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

A. what

B. this

C. that

D. which

答案：

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。