根据句子意思选出正确的单词。
How long  How old   How far   How much   How many 
1.—________ is it from Beijing to Tianjin?—It's about 125 kilometres.   
2.—________ is your mother?—She is forty years old.   
3.—________ is the Great Wall?—It's about 6000 kilometres.   
4.—________ students are there?—There are forty-­five.   
5.—________ is this book?—It's five yuan.