关于声现象，下列说法中正确的是（  ）

古尔邦节，6位朋友均匀地围坐在圆桌旁共度佳节．圆桌半径为60cm，每人离圆桌的距离均为10cm，现又来了两名客人，每人向后挪动了相同的距离，再左右调整位置，使8人都坐下，并且8人之间的距离与原来6人之间的距离（即在圆周上两人之间的圆弧的长）相等．设每人向后挪动的距离为x，根据题意，可列方程                                                 （　 　）

A: Hey, Peter. ____ 
B: I argued with my best friend. ____ 
A: Well, you could write him a letter.
B: I don't think so. I don't like writing letters.
A: ____ .
B: No, I don't want to talk about it on the phone.
A: Well, you should say you're sorry.
B: Yes, I know I should, but it's not easy.
A: Hey, I know. You could give him ____  a ball game.
A: Well, that's a good idea, but ____ .

小明用凸透镜、蜡烛、光屏和刻度尺等器材，探究凸透镜成像的规律。

立定跳远时，如果起跳板上有沙粒，起跳时就会感觉很滑，影响跳远成绩，这是因为起跳板上的沙粒把滑动变为____，使摩擦力大大减小。