The energy source on Voyager 2 is not a nuclear reactor, in which atoms are actively broken apart;
rather a kind of nuclear battery that uses natural radioactive decay to produce power.
(A) apart; rather
(B) apart, but rather
(C) apart, but rather that of
(D) apart, but that of
(E) apart; it is that of

OA is B

Could we use here Not X but Y ? Are both interchangeable ?