题目

Losing a baby tooth can be fun for a kid. You can put it under your pillow and look forward to finding some money there in the morning. And then you can watch your new permanent(永恒的) tooth grow in its place. But if a permanent tooth falls out, that’s a problem. There isn’t another one under the gum(齿龈)that can replace it. Unfortunately, this happens to some older people. And if permanent teeth fall out-or need to be pulled-an older person might need dentures. Dentures, known as false teeth, are a set of replacement teeth for any teeth that are missing. There are partial dentures,大气水平运动的根本原因是 [     ]A、水平气压梯度力　　　　　 B、气压差异 C、冷热不均　　　　　　　　　　 D、地转偏向力