A

(Mr. Smith and Bob are in a store.)

Man: What can I do for you?

Mr. Smith: I want a sweater for my son.

Man: What color does he like?

Mr. Smith (to Bob): What color .do you like?

Bob: I like blue.

Man: Do you like this one?

Bob: Yes. It looks nice. How much is it?

Man: 80 dollars.

Mr. Smith: It's too pricey.

Man: Look at this black one. It's on sale. It's only 30 dollars. This green one is 50, and that brown one is 40.

Mr. Smith: Mmm…… I think the black one is nice, too.

Bob: But I don't like black. I like blue.

Mr. Smith: OK, we'll take the blue one.

Bob: Thank you, Dad.

根据材料内容选择最佳答案

阅读材料，回答下列问题。

材料 如大气中存在过量的由燃烧产生的SO₂和NO₂则大气降水的pH值低于5.6以下，称为酸雨。

  酸雨属于二次污染物，其环境影响是区域性的。目前在世界范围已形成几大酸雨区，我国西南地区酸雨污染严重，频率高，单场雨pH值最低曾达3.35(重庆)和3.41(贵阳)，并已蔓延到广东、广西等省。

明理导行
随着网络的发展，如今校园生活越来越丰富多彩，某校园网的论坛上开设了一个“生活杂谈”，点击率居高不下，下面是部分同学的留言：
       

 请你结合所学知识，对小梅、小军两人的言行进行简要的评析。 

潜水艇的上浮和潜水是靠改变　____ 　来实现的。