it takes many years to gain the skills of an expert.
A. It is well known to us that
B. As is known to us that
C. As we know that
D. That is well known to us
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(答案→)A. It is well known to us that
B. As is known to us that
C. As we know that
D. That is well known to us
解析： 考查主语从句：句意：要花费很多时间才能获得专家的技能这是众所周知的。这里it是形式主语，that引导的是真正的主语从句，如果是B和C项，应该是把that改成逗号，变成非限制性定语从句，D项错在that不能走形式主语。选A。