请您对以下题目给予具体解释,为什么这样做!以便下次不会出错,

请您对以下题目给予具体解释,为什么这样做!以便下次不会出错,
1._____is easy for us to do homework in twenty minutes.
A That B What C It D This
2.I have never been to Thailand ,but I love __________food a lot
A Thai B Thai land C Thai 's D Thai land's
3.Forests and land are important _____animals and people ______many ways
A to ,in B for ,in C to,by D for ,by
4.---How long will this superstar stay in Shanghai?
--She'll stay here _______the end of August
A at B until C after D by
There may be enough food.(改为否定句)
There _______ _________ be enough food.
There was a lot of traffic in our city in the past(改为否定句)
There_____ _______traffic in our city in the past