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同学你好~
There are 7 continents and 4 oceans, as is known to all.
As is known to all, there are 7 continents and 4 oceans.
There are 7 continents and 4 oceans ,which is known to all.
这三种说法都是正确的。
但是
Which is known to all, there are 7 continents and 4 oceans.
这种说法是错误的。

as引动非限制性定语从句，可位于主句前或者主句后，which引导的非限制性定语从句只能位于主句后。
祝学习愉快！