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I wonder both of the below can be used.

Biological organization is based on a hierarchy of structural levels, __________ on the levels below it.

1.
Biological organization is based on a hierarcy of structural levels,
and each of them build on the levels below it.
--> Biological organization is based on a hierarcy of structural levels,
each of which build on the levels below it.
--> Biological organization is based on a hierarcy of structural levels,
each of which building on the levels below it.

2.
Biological organization is based on a hierarcy of structural levels,
and each level builds on the levels below it.
--> Biological organization is based on a hierarcy of structural levels,
each level building on the levels below it.

Thanks

Biological organization is based on a hierarcy of structural levels,
and each of them build on the levels below them.
--> Biological organization is based on a hierarcy of structural levels,
each of which build on the levels below it. -Correct.
--> Biological organization is based on a hierarcy of structural levels,
each of which building on the levels below it. -gobbledegook

Biological organization is based on a hierarcy of structural levels,
and each level builds on the levels below it. -Correct
--> Biological organization is based on a hierarcy of structural levels,
each level building on the levels below it. -Correct

heyiamhere wrote:I wonder both of the below can be used.

Biological organization is based on a hierarchy of structural levels, __________ on the levels below it.



Thanks

As Stephen, plus:

--> Biological organization is based on a hierarcy of structural levels,
each building on the levels below it.