Today's technology allows manufacturers to make
small cars more fuel-efficient now
than at any time in their production history.
A.small cars more fuel-efficient now than at any time in their
B.small cars that are more fuel-efficient than they were at any time in their
C.small cars that are more fuel-efficient than those at any other time in
D.more fuel-efficient small cars than those at any other time in their
E.more fuel-efficient small cars now than at any time in
The sentence, as stands, is confusing. It implies that the 'small cars' are now more fuel efficient than they were in the past. That is the manufactures took an old small car and said, "hey let's make this more efficient." Clearly, however, the small cars today are different from those in the past. Notice, how I use the word 'those.' It refers to the other 'small cars' in the past. For this reason, we can omit (A), (B).
Notice, how (D) and (E) change the meaning of the original sentence. Both imply that manufacturers are making a great number of fuel efficient cars, and not cars that are more fuel efficient. To illustrate, saying that you have increased the number of cars that get 50 km/gallon is different from saying that you now make more fuel efficient cars that can get. 60km/gallon.
This leaves us with (C).
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