根据短文内容，选择正确答案。

It's Sunday. There are many students in the park. They are picking up useful thing.

Mike and Li Lin are picking up bottles. Jill and Liu Xin are putting them into a bag. How many bottles are there? There are forty-two.

Mary and Meimei are picking up paper. Ann is putting the paper into a bin（垃圾箱）. How much paper is there?

About half a kilo.

The students are doing a good job, and they are very happy.