句子补全应该是Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than (it had been)in any previous election. it指代 proportion
所以A错在时态上,C则比较对象不对,因为they只能指代costs。这确实类似于OG198 Although Napoleon's army entered Russia with far more supplies than (supplies) for any previous campaign, it had provisions for only twenty-four days. 根据OG198的讨论,for前省略的是supplies,虽然OG解释中有这么一句话:If than is followed by a clause referring to army, the subject of that clause must be singular (it). Furthermore, the verb of that clause will need to be in the past perfect form (had had) because it refers to a time before the simple past of entered. 但注意,解释的开始有If表示假设,表前提。