grammar

[BobK]

Test constructors these days are hung up on "parallel structure," the opposite of elegant variation.

The answer is b because the sentence compares Newton's to Einstein's [contributions].

...

Thanks.

b

[BobK]

Hello, BobK and mykwyner !!

Many thanks for your explanation with note.
Now I feel I have escaped from the last headache !!! ...

Best Rgds,

You're welcome.

Incidentally, "that of N's" isn't always wrong; you have to balance the number of possessives before and after the 'as' or 'than':

Einstein's theory is more complex than Newton's. [1 possessive before and after].

The publication of Einstein's theory was more problematic than that of Newton's. [2 possessives before and after]

The second is rarer; but it's possible.

b