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.