有理数a、b在数轴上的位置如图所示，则下列各式错误的是（　　）

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人们经常听到这样的说法“这种食物好，容易消化；那种东西难消化”。食物为什么会有“好消化”或者“难消化”的差别呢？

有效的消化取决于两个条件：一是有充足的消化酶及胆汁；二是消化酶与所能消化的食物充分接触。

淀粉需要在淀粉酶作用下被分解才能被消化吸收。纤维素不能被淀粉酶分解，人类没有相应的酶，所以它们不能被消化吸收，会原封不动地经过小肠进入大肠。

脂肪需要脂肪酶才能被分解吸收。但是脂肪不溶于水，好在有胆汁把脂肪变成小颗粒，脂肪和酶的接触面积大大增加，更容易被消化。

有的食物成分之间能够发生反应生成沉淀，影响消化，最有名的就是“柿子不能与螃蟹同吃”。这其实是因为柿子中含大量单宁，与螃蟹中的蛋白质反应生成不溶物，这种不溶物能够抵御胃肠内蛋白酶的进攻，导致无法消化。

在我们的肠道中有很多微生物可以帮助我们分解食物。它们就是通常所说的“益生菌”。这个家族中的主要成员有乳酸菌、双歧杆菌、酵母菌等。由它们组成的“复合活性益生菌”，是一个充满战斗力的集体。

当然，有很多“减肥食品”实际上正在刻意地追求“难消化”。这些食物吃进去，满足了“食欲”和“胃口”，但很难被消化吸收，从而避免增重。

    Wouldn't it be funny if your mum said to you, "It's time for bed, honey. It's almost November!" Well, if you were a baby bear, your mother might let you know it's time to go into a deep sleep around October.

    That "deep sleep" is called "hibernation (冬眠)". When it gets cold, bears have to go into hibernation. By taking this long sleep, they can get through the cold months when the food, like berries (浆果) and nuts, cannot be found easily.

    Have you ever heard someone say that they are as hungry as a bear? Bears need to eat a lot of food before hibernating. They eat so much that they may put on forty pounds of fat in a week. During the winter "bears will live off the fat stored in their bodies. Bears are able to do that because their bodies use a lot less energy during hibernation. Their heartbeats slow down, their breathing slows down, and their body temperature drops.

    If you were going to sleep all winter, wouldn't you want to be in a safe and protected place? That is what a bear wants, so it looks for a cave to sleep in. It makes a bed using grass, leaves and twigs (嫩枝). The bear usually starts to hibernate as early as October and doesn't wake up until April or May. By the time the bear wakes up, it will have lost much of the fat. But spring time will bring much food to enjoy until it is time to hibernate again in the autumn.