The energy source on Voyager 2 is not a nuclear reactor, in which atom

This one is short and sweet. Nice way to end the week, no?


Hopefully, the semicolon jumps off the page at you immediately. In general, semicolons need to separate full, independent clauses. (There's also a rare usage in which semicolons can separate items in a list of individual items that already contain commas, but that's unlikely to cause you much pain -- an official example is available here.) In this case, the semicolon makes no sense: "rather a kind of nuclear battery..." is not a full, independent clause. (A) is out.


This looks fine. "The energy source is not a nuclear reactor..., but rather a kind of nuclear battery." It's a classic "not X, but rather Y" sort of construction, and it makes logical sense: we're clarifying that the energy source is something other than what we might expect. And there are no semicolon issues. Let's keep (B).



If you've ready our post on the GMAT's many uses of the word "that", then the "that of" construction should leap off the page at you. "That" is used as a singular pronoun here, so your job is to figure out what the singular pronoun refers back to... and then reread the sentence to make sure that it actually makes sense.

OK, so what are our options for singular referents for "that"? Well, there's "the energy source" or "a nuclear reactor." Let's try both of those, and see if either of them work:

• "The energy source on Voyager 2 is not a nuclear reactor, in which atoms are actively broken apart, but rather the energy source of a kind of nuclear battery that uses natural radioactive decay to produce power." --> Huh? That's just weird and redundant, and doesn't really make sense. We could say that the energy source IS a nuclear battery, but "the energy source is... the energy source of a kind of nuclear battery" really doesn't work.
• "The energy source on Voyager 2 is not a nuclear reactor, in which atoms are actively broken apart, but rather a nuclear reactor of a kind of nuclear battery that uses natural radioactive decay to produce power." --> Nope, that's even worse.

So (C) is out.


(D) has exactly the same error as (C). Removing the word "rather" doesn't change the fact that the pronoun is completely nonsense.


(E) has the same mistake as (C) and (D). And we can probably argue about the "it", too. But either way, (E) is gone, and (B) is the best of the bunch.