答：选C解析：本题考查代词的用法。四个选项中，只有that可以代替句单数可数名词或不可数名词，以避免重复。如：The output of steel this year has increased by 20% compared with that in 1992. 今年钢产量比一九九二年增长了百分之二十。（此句中，后面的that指代前面的output这个不可数名词）。又如：The book on the desk is not so interesting as that in the desk.(此处的that指代前面的book这个单数可数名词，相当于the one)。本题句意：他们必须考虑的最重要的问题之一就是公共卫生问题。如果还原这...
B one One of the most important questions they had to consider主语，其中they had to consider为questions的定语从句（非表语从句哦） was系动词 _one_of public health表语译：他们不得不考虑的重要问题之一是公共卫生问题...
