3道语法题（SAT）

题目

3道语法题（SAT）
1.For the past hundred years or more,Yellowstone National Park "was"a kind of sociological laboratory in which North Americans have been exploring the meaning of the national park concept.
文中was的用法是错误的,请问正确答案是?为什么?
2.Introducing new ideas and replacing old ones "is" always a highly controversial matter.
文中的is用法是正确的.不过我不明白的是,主语不是plural麽（introducing ...replacing)?
3.The often conflicting reports of different polling organizations "make it difficult" to predict which of the two candidates is more likely to win the election next year.
文中的引号处用法正确.我不明白的是我认为引号中的it指代不明...不过看来我理解错了?请问it指代什么?make it difficult是固定用法麽?

答案

1. was 改为 has been, 注意时间状语：for / during/ in / over the past + 时间段,句子谓语动词要用现在完成时.eg: I have made much progress in the past three weeks.
2.此句中虽然有and,但是它并列的是一个主体的两个方面, 如bread and butter 黄油面包,不能理解为 黄油和面包.
3.it 作形式宾语,代替后面的不定式 to predict .的内容.