We live in the computer age. People use computers to do much of their work. But few people know how to look after them. Remember the following when you use your computer:
    ① Keep your computer in a cool ,dry room. Too much or too little heat is bad for computers. Damp (潮湿) is also bad for them.
    ② Do not let people smoke near your computer. Smoke of all kinds is very bad for many parts of a computer.
    ③ Do not eat or drink when you are near your computer. Pieces of food and a little water in the keyboard are also bad for a computer.

“天生我才必有用”这句话强调         （）                        
①自己天生就是有用之才  ②每个人认识和发挥自己的优势和长处,树立自信心
③每个人都可以自然成为有用之才  ④只要努力，每个人都可以成为有用之才 

$(3\sqrt{x}-\frac{1}{\sqrt{x}}{)}^n$的展开式中各项系数之和为64，则展开式的常数项为（  ）