3道语法题（SAT）1.For the past hundred years or more,Yellowstone N

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?