He must be from Africa,__________ can be seen from his skin.

which或者as
which 引导非限制性定语从句,指代前面这句话.
as 确实可以引导方式状语,比如I did the work as my father taught me.
但是,这里是定语从句,只不过先行词不是一个名词或者代词,而是真个句子
在非限制性定语从句中,which和as可以引导定语从句修饰整个句子,注意that,where,when,why等都不行.
举个列子,They did rather badly in the maths exam,which was not what I had expected.
这前面整个“They did rather badly in the maths exam”
是我所没意料到的
还有更简单的例子,也就是as we all know结构
As we all know,china is a developping country.
这里we all know 的内容就是“china is a developping country”这整一句话.
补充一下,as引导方式状语时,从句式完整的句子,而引导定语从句时,则从句缺少成分,比如As we all know,缺少的就是know 的宾语,而宾语其实就是主句“china is a developping country”

it / that 都可以
意思是,他应该来自非洲,这可以从他的皮肤看出来.

填as
这是定语从句，应该用as。
他一定来自非洲，这从他的肤色就看的出来。

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which
非限制性定语从句，因为中间是逗号，是一个句子
他一定是来自非洲，这从他的肤色可以看出