is known to us all is the sun is bigger than the earth.A It; that B What ; that C What ;

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解析：考查主语从句和表语从句。本题第一空what is known to us all是一个主语从句，what引导起主语从句并在句中做主语。第二空that引导的是表语从句，that在句中不充当任何成分，只起引导和连接作用。句意：众所周知的事情是太阳比地球要更大。故B正确。