A的it不能指代a greater proportion;而且accounted和was并无法表达时态的不同,恰恰相反,两者用了同一个时态-一般过去时;其实,这里的比较可以被看作,the proportion in the campaign of 1992 was greater than (that) in any previous election
我觉得it可以指代proportion,但it指代的东西是特指,不仅指代了porportion,而且是proportion in 1992的所有特征,又怎么能再具有 in any pervious election的特征。所以这里it是错的。 而that是泛指,只指代porportion,且被指代的proportion不具备前面的性质。。 我觉得A,主要错在了多余的动词补出。
赞同:“Soaring television costs accounted for a greater proportionin the presidential campaign of 1992than (they had accounted for) in any previous election.”