Hi,
I am confused with this one.

Q.
In recent years cattle breeders have increasingly used crossbreeding，in part that their steers should acquire certain characteristics and partly because crossbreeding is said to provide hybrid vigor.

(A) in part that their steers should acquire certain characteristics
(B) in part for the acquisition of certain characteristics in their steers
(C) partly because of their steers acquiring certain characteristics
(D) partly because certain characteristics should be acquired by their steers
(E) partly to acquire certain characteristics in their steers

Confusion:
While checking for parallelism here, should one check for Partly because or partly?
The answer given is confusing me i.e. E.

Please clarify.