每日一题：

一共三道题，初中浮力题（图一）
高中数学题（图二）英语四级题（选项见投票）

高中数学：
（1）一共6个不同的区域，用4种颜色涂刷，每两个区域不能相邻，一共多少种情况？

（2）如果有6种颜色呢？

英语四级：
You might think of your teeth as tools, like built-in knives and forks. But if they are mere tools, why do teeth feel pain and wouldn't it be better if they could just__26__under any conditions? In spite of our__27__
discomfort, it turns out there's a good reason our teeth are so sensitive.Tooth pain is a__28__mechanism that ensures when a tooth is being damaged.We'll notice and do something about it.

If you eat something too hot or too cold, or if the tooth is worn down enough where the tissue __29__ is exposed, all of those things cause pain, and then the pain causes the person not to use that tooth to try to protect it a little bit more. So it's really a protective mechanism more than anything else. If teeth didn't feel pain, we might__30__ to use them in situations that damage them. And for humans, damaging31 teeth is a problem because, unlike crocodiles, we can’t__32__them.

Teeth have three layers, only one of which—the innermost layer of the tooth hurt, as that layer of the tooth__33__ both blood vessels and nerves. Pain is the only feeling to which the nerves in that layer respond. Whereas people with tooth sensitivity may complain, for example, of tooth pain__34__ by heat or cold, the nerves in the inner layer don't sense temperature.Rather, they feel pain, which may be__35__ with, say, drinking something very cold. http://t.cn/A6Rw9uQK