is known to all is that effective measures should be taken to deal with the problem of the over-exploiting of our resources.
A. What
B. It
C. As
D. Which
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(答案→)A. What
B. It
C. As
D. Which
解析：考查名词性从句。分析题干结构“ is known to us all”是主语从句，从句中缺少主语，what 引导主语从句，在从句中作主语。如果选 it，需去掉 all 后的 is，如果选 as需去掉 is that。句意为“众所周知，面对过度开发资源的问题，我们应该采取有效的措施。” as 作为关系代词引导的非限制性定语从句位于主句之前时， It从句与主句之间要用逗号隔开。作为形式主语将 that 引导的主语从句后置时，that 常常不能省。选A。