172. 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

Choice B, the best answer, follows an idiomatic form of expression for paired coordinates--not X, but rather Y;

here rather is optional but preferable because it helps estab­lish a contrast between the two types of energy source. Choice A incorrectly uses a semicolon rather than a coordi­nating conjunction (but) to connect the coordinate parts; a semicolon should be used to join independent clauses. In choices C, D, and E, that of has no grammatical referent and thus produces illogical and incorrect sentences.

我认为taht在句中确无指代对象，a kind of nuclear battery是与a nuclear reactor相平行的，taht在这里是一个多余的单词.

從解釋看來, OG是把that of視為一個單位,

前面要有結構相同的單位才能指代

大家再討論.

not a nuclear reactor, ... , but rather a kind of nuclear battery ... 比較對象平行，加上that of反而沒有指代對象!!