中国人口问题的主要矛盾是（    ）

完成下列作图：

（1）图甲是一杠杆图，试画出作用在杠杆A端使杠杆在图示位置平衡的最小动力的示意图并画出该力的力臂．

（2）图乙是未装配好的滑轮组，请在图中画出滑轮组的绕绳方法，要求使用该滑轮组提升重物时最省力．

若函数$f\left(x\right)=-a·2^x$与$f\left(x\right)=4^x+a+1$的图象有交点，则a的取值范围是（   ）

                                                                                Just a Quick Nap 只要打个盹儿
      Do you feel a little sleepy after lunch? Well, that's normal. Your body naturally slows down then. What should you do about it? Don't reach for a coffee! Instead, take a nap.
       It's good to have a daily nap. First of all , you are more efficient（有效的） after napping . You remember things better and make fewer mistakes. Also, you can learn things more easily after taking a nap. A nap may increase your self-confidence（自信） and make you more active . It may even cheer you up. But, there are some simple rules you should follow about taking a nap. First, take a nap in the middle of day, about eight hours after you wake up. Next, a 20-minute nap is best. If you sleep longer, you may fall into a deep sleep. After waking from a deep sleep, you will feel worse. Also, you should set an alarm clock. That way, you can fully relax during your nap. You won't have to keep looking at the clock so you don't oversleep.
       Now，the next time you feel sleepy after lunch , don't get stressed . Put your head down, close your eyes, and catch forty winks.