一道英语选择题The moon travels around the earth once every month,__is known to everyone.

which本句也可以用，是非限制性定语从句。但as本句当然也已使用。
因此，本句which,as兼可。
which不可以用于句首，但是若要选一个则选as,因为一般说as is known to everyone,as is known to all等等。
回答完毕。

The moon travels around the earth once every month,_as_is known to everyone.
you are right.

英语中的习惯问题，一般这种常识性的表述，人所共知的事，英文中就是as is known to everyone，这句话也可以说成：As is known to everyone that the moon travels around the earth once every month.

是吗?我怎么没听说过？此句中as 似乎可以换成which呀……