Everyone knows that exercise is important. We all need to exercise. There are many ways to exercise. You can walk, run, swim, skate, or play ball games, but you should remember to exercise in the right ways.

        ①    Before you do any exercise, make sure to warm up for at least five minutes. During warming up, you should extend all parts of your body.

        ②      It's best to exercise twice each week. Thirty minutes each time is enough. Try all kinds of things until you find one, two or even three sports that feel right for you.

        ③    There are a lot of equipments (设备) at fitness centers. The equipments will help exercise your arms, legs and other parts of your body to make you fit. Some people buy equipments for their homes. But it is very expensive.

        ④     If you get hurt when you are doing exercise, don't play again until you are well.

A．在量筒中倒入适量的水，此时液面的示数为V₁

B．把游码放在标尺的零刻度线处，调节横梁上的螺母，使横梁平衡

C．将小矿石用细线系好后浸没在量筒内的水中，此时液面示数为V₂

D．将小矿石放在左盘中，在右盘中增减砝码并移动游码直至横梁平衡

E．把天平放在水平桌面上

正项递增等比数列{$a_n$}中，$a_3{a}_7{a}_8{a}_{10}=81,a_5+a_9=\frac{51}{4}$，则该数列的通项公式$a_n$为（   ）