OG上的一道语法题

tianlingfrl

Whereas a ramjet generally cannot achieve high speeds without the initial assistance of the rocket,high speeds can be attained by scramjets,or supersonic combustion ramjets,in that they reduce airflow compression at the entrance of the engine and letting air pass through at suppersonic speeds.
替换答案是
scramjets,or supersonic combustion ramjets,can attain high speeds by reducing
我想问下，句尾and后面连接的letting和前面的reducing是并列的么？为什么这里省略了介词by？XDF老师笔记是说的有介词的并列短语要尽量保留介词，那么什么时候确定保留什么时候是可以省略的啊？

jtrchina

Since only one "ing"exists in the whole sentence,there would be no other "ing" to parallel with reducing,thus making the parallelism clear and avoiding alternatives that make parallelism ambiguous.In this case,preposition can be omitted.
For exemple,a sentence like:
"I write this posts that can help you get improved and make the point clear."
In this sentence,you can either say that I have done 2 things:write the post and make the point clear.However,what I want to express is that it is my posts that make the point clear.An omission of "that" make the meaning of this sentence ambiguous.If I had written "I write this posts that can help you get improved and that make the point clear.",you would never get confused,right?
In all,whether to omit the preposition depends on whether meaning of sentence will be ambiguous after the omission.
Hope it helps!