心脏内以及心脏和动脉之间的瓣膜，使血液的流动只能是（　　）

某学校地理兴趣小组做了如下实验：做两个相同规格的玻璃箱，甲底部放一层土，中午同时把两个玻璃箱放在日光下，十五分钟后，同时测玻璃箱里的气温，结果发现底部放土的比没有放土的足足高了3℃．据此回答下题．

如图描述了解放战争期间某一阶段的战略态势，图中的“尖刀”表示的是（　　）

A: Hello, Jim. ____?

B: I have a pain in my hand, doctor.

A: ____?

B: Since Sunday. It's very painful.

A: ____?

B: I was playing basketball with another team.

A: ____? Did you have a fall?

B: No. Nothing special happened.

A: Well, Let me check you. OK. You don't have a serious problem with your hand. You just need to have a rest.

B: I'd like to have a rest. But I will have exams tomorrow. My hand is so painful that I'm afraid I can't go to school. ____?

A: Here is your medicine, but you'd better ask for leave.

B: Thanks for your advice, doctor. The exams are quite important for me, so I have to.

A: Well, I hope you can do well in your exams, Jim.

B: Thank you, doctor.