功率因数校正

二尖瓣狭窄患者可听到的最重要的杂音是（）

A.心尖区收缩期吹风样杂音

B.心尖区舒张期隆隆样杂音

C.主动脉瓣区舒张期叹气样杂音

D.肺动脉瓣区舒张期吹风样杂音

E.胸骨右缘第2肋间响亮而粗糙的双期杂音

自北宋以来，历朝反对火葬的重要原因是什么？

设x²+y²+z²=4z,确定函数z=z(x，y)，求．

阅读理解.

     America has long been considered the land of opportunity(机遇)by those from

other countries. Americans, too, believe that the United States gives almost limitless

opportunity for those who want to open business on their own.

     Today, Americans are still fond of trying their hand at becoming small business

people, even though only one out of two survives in the first two years. Many of these

people start their business for the wrong reason: to get away from the paper work of

their present jobs or to exchange the responsibility(责任)of their present jobs for free

life styles. But more, not less paper work and responsibility come with the ownership

of a small business.

     Not all small businesses succeed. Fifty percent of the 4,550,000 that start in the

United States ever fail(失败). Still ninety-five percent of businesses in the U.S. can

be regarded as small, and these businesses altogether produce forty percent of

America's total national products.

1. According to the passage, which of the following statements is TRUE?______

A. People from other countries and Americans think there are a lot of

opportunities in theU.S.

B. Only the Americans think the U.S. can give them many opportunities.

C. Both people from other countries and Americans believe they have many

opportunities in the U.S so that they can do whatever they like.

D. People from other countries and Americans all think the U.S. the place where

they can realize their dreams.

2. From this passage, we know that ____.

A. many people from other countries think that in the past there were many

opportunities in the U.S. but not now

B. now Americans don't believe the U.S. can give them a lot of opportunities

C. people from other countries still think they have more opportunities in the

U.S. than in their own countries

D. Americans think they have more opportunities to start their own business than

those from other countries

3. According to this passage, many American people like to start their own

business because ____.

A. they want to become rich soon

B. they not only dislike their present job but also want a free life style

C. they don't want to do paper work

D. they want to have more chances

4. From this passage, we can get the idea that ____.

A. it is easy to do business successfully in the U.S.

B. it is easy to start a business but hard to make it successful

C. there are more big businesses than small businesses in the U.S.

D. only big businesses can succeed in the U.S.

调制型放大器的主要调制器件是振荡变流器。