Meaning is key to solve this question. Grammar alone is not enough.
In recent years, troubled airlines have exchanged their aging, inefficient fleets for a smaller number of efficient new airplanes,
in part because high fuel costs should be lowered and repair costs should be reduced and partly because low entry barriers for new carriers have created intense competition.
(A) in part because high fuel costs should be lowered and repair costs should be reduced
Wrong. - Not parallel: in part vs partly
- Incorrect meaning: The intended meaning is that the trouble airlines exchanged old fleets to lower
THEIR costs. The sentence, however, says they exchanged old fleets to new ones because
high fuel costs (in general) should be lowered. That's does not make sense.
If high fuel costs will be lowered, these troubled airlines will NOT need to exchange their old fleets. They exchanged old fleets to new ones because they need to lower THEIR high fuel costs to create competitive advantage.
(B) in part that high fuel costs might be lowered and repair costs might be reduced
Wrong. - Not parallel: in part vs partly
- Incorrect meaning: same as A.
(C) partly because high fuel costs should be lowered and repair costs should be reduced your answer
Wrong. SHELL GAME.If you just base on grammars and picked C, you got the sentence wrong because C conveys WRONG meaning. See explanation on A.
(D) partly for the lowering of high fuel costs and the reduction of repair costs
Wrong. - we should use verb rather than the noun form of it. Verbs always convey better meaning and are more precise.
(E) partly to lower high fuel prices and reduce repair costs
Correct.- Parallelism: partly || partly
- Correct meaning: exchange old fleets to new ones TO lower costs.
Hope it helps.