地球是一个球体，它的平均半径和表面积分别为（　　）

阅读下列对话，选择正确的答案。
Miss Wang: Good morning, boys and girls! I’m Wang Li. And I’m your（你们的）English teacher（教师）.
Boys and girls: Good morning, Miss Wang!
Miss Wang: This is Helen. She is English（ 英国人）.
Helen: I’m Helen. Nice to meet you.
Miss Wang: Are you Dale?
Frank: No. I’m Frank.
Miss Wang: OK. Now, Frank, what’s this in English?
Frank: It’s a ruler, Miss Wang.
Miss Wang: What color is it?
Frank: White.
Miss Wang: Right. And what color is this pen?
Frank: It’s blue. I like（喜欢）blue, because（因为）I like the sky（天空）.
Helen: No. The pen is green. And I like green. Look! My jacket is green.
Miss Wang : Great!

甲同学设计了如下实验装置验证一氧化碳的部分性质并验证产物．实验时，在点燃B处酒精灯之前先通入一氧化碳排出装置中的空气，然后继续实验．