SAT grammar,有关平行结构,

题目

SAT grammar,有关平行结构,
A radio system consists of a means of transforming sounds into electromagnetic waves,(transmitting the waves through space,and transforming) them back into sounds.
这是个正确的句子.
可是据我所知,平行结构中介词应该是要带着平行的,比如这里应该是of transmitting the waves...和of transtorming them...正确答案里为什么没有这个 of

答案

You can as you have believed, but the vast majority of people would not. Only when the prepositions are different do we need to keep them. Obviously, this is not the case so far as this sentence is related.