问题 问答题 简答题

V形球阀有何特点?

答案

参考答案:

V形球阀又称V形切口球阀,阀芯为1/4球壳,开有V形缺口,其优点是:

①流通能力大,比普通阀高2倍以上;

②控制特性好,为等百分比;

③可调范围大,可达300:1;

④具有剪切作用,能严密关闭,适用于浆料、纤维状流体场合。

单项选择题
单项选择题

In the absence of optimism, we are left with nothing but critics, naysayers, and prophets of doom. When a nation expects the worst from its people and institutions, and its experts focus exclusively on faults, hope dies. Too many people spend too much tie looking down rather than up, finding fault with their country’s political institutions, economic system, educational establishment, religious organizations, and—worst of all—with each other.

Faultfinding expends so much negative energy that nothing is left over for positive action. It takes courage and strength to solve the genuine problems that afflict every society. Sure, there well always be things that need fixing. But the question is, do you want to spend your time and energy tearing things down or building them up

The staging of a Broadway show could illustrate my point. Let’s say a new production is about to open. A playwright has polished the script, investor have put up the money, and the theater has been rented. A director has been chosen, actors have been auditioned and selected, and the cast has been rehearsing for weeks. Set, lighting, and sound engineers have been hard at work. By the time opening night arrives, nearly a hundred people have labored tirelessly—all working long hours to make magic for their audience.

On opening night, four or five critics sit in the audience. If they pan it, the play will probably close in a matter of days or weeks. If they praise it, the production could go on for a long and successful run. In the end, success or failure might hinge on the opinion of a single person—someone who might be in a bad mood on opening night! What’s wrong with this scene In one sense, nothing. Critics have a legitimate role. The problem arises when we make critics our heroes or put them in control of our fate. When we empower the critic more than the playwright, something is wrong. It is much easier to criticize than to create. When we revere the critics of society, we eventually become a society of critics, and when that happens, there is no room left for constructive optimism.

According to the author, critics usually().

A. ignore minor imperfections

B. overemphasize flaws

C. see both sides of a coin

D. pin their hopes on improvements