The majority of scientists believe that to reduce and stabilize atmospheric concentrations of carbon dioxide, emissions must be cut at the source by fossil fuels that are burned more efficiently and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether.


(A) fossil fuels that are burned more efficiently and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether

(B) fossil fuels that are burned more efficiently and, in some cases, those that are replaced altogether with alternatives such as hydrogen fuel cells

(C) burning fossil fuels more efficiently and, in some cases, by replacing fossil fuels altogether with alternatives such as hydrogen fuel cells

(D) more efficiency in the burning of fossil fuels and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether

(E) the more efficient burning of fossil fuels and, in some cases, by fossil fuels that are replaced altogether with alternatives such as hydrogen fuel cells

The sentence has 2 corrections;

1. the clauses that are parallel do not agree on the verb form:
"burned" and "can replace" are not parallel.

2. the modified parts of the two sentence fragments are not well formed.

C - corrects this using "ing" form of the verb and placing the modified fragments in context i.e "burning fossil fuels" and "replacing fossil fuels"

I picked C because the word before the underlined portion is "by", which is a preposition and should be followed by a noun or object. That eliminates D & E, and A and B do not follow the rules on parallelism which leaves us with C.

This sentence needs a common construction: ‘by + verb-ing’. 'By' is used when you describe the method used to accomplish (HOW) something - exactly what is done in this sentence. The sentence is explaining how the states are going to accomplish the stated goal.

If we say:

You should study daily. By doing so, you will improve consistently.

Or

One can’t become smarter by facing the same challenges every day.

A and B are wrong because emissions can’t be cut by fossil fuels … rather they can be cut by more efficient burning of fossil fuels.

D is not parallel; ‘by more efficiency’ is not parallel to ‘alternatives (fuels)’.

E is not parallel; ‘the more efficient burning’ is not parallel to ‘fossil fuels’.

C is perfectly parallel … by burning || by replacing.

C: correct.
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Thanks & Regards,
Anaira Mitch

https://www.youtube.com/watch?v=R9rcv8rN0EM&list=PL0LGZMx8pP21Jx8Er_L5V-HfbPn9DKY7M&index=12

54:35