某交流电电源电压的瞬时值表达式为u=6sinl00πt（V），则下列说_爱下问搜题

搜题请搜索小程序“爱下问搜题”

问题选择题

某交流电电源电压的瞬时值表达式为u=6sinl00πt（V），则下列说法中正确的是（　　）

A．用交流电压表测该电源电压时，示数是6V

B．用交流电压表测该电源电压时，示数是6V

C．用此电源给电磁打点计时器供电时，打点的时间间隔为0.01s

D．把标有“6V 3W”的小灯泡接在该电源上时，小灯泡将被烧毁

答案

答案：A

多项选择题

起源于胞中的奇经是（）。

A.任脉

B.阴维脉

C.带脉

D.督脉

E.冲脉

阅读理解

阅读理解。

As bananas ripen, the green skin turns yellow gradually. Chemical changes occur in the flesh of the fruit
as well: starch (淀粉) changes to sugar; pectin (果胶) breaks down, losing its stiffness; and the flesh softens.
Is ripening fruit a self-contained system, or is the rate of these changes affected by the environment? We
designed the experiment below to see if different atmospheres, produced by different packaging, affected the
rate of ripening. You will vary the atmosphere surrounding unripe bananas and observe the effect on the ripening
process.

     Now set up the bananas in their
environments as follows:
     1. Put two green bananas in a paper bag
and fold the top over to seal out the air.
     2. Put one green banana and the very
ripe banana in the other paper bag and fold
over the top.
     3. Put two green bananas in a plastic
bag and seal it.
     4. Wrap (包) one green banana tightly
in a plastic bag.
     5. Leave one green banana exposed to
the air.

Leave the fruit alone for four or five days to ripen. Ripening fruit "breathes". This means that it takes up
oxygen and gives off CO₂. Oxygen is extremely important and necessary for the chemical reactions involved
in ripening. In addition, ripening fruit gives off another gas, called ethylene (乙烯). Not only is ethylene a product
of ripening fruit, it also promotes the further ripening of the fruit.
Paper bags tend to keep the ethylene in, but they allow oxygen and ethylene to pass through slowly. Plastic
bags do not allow the free flow of oxygen or ethylene. In this experiment, the green banana in paper bag with
the ripe banana should ripen most quickly. The green bananas in the paper bag should ripen faster than the
bananas in the plastic bag. The banana left exposed to air has an unlimited supply of oxygen, so it will turn brown
most quickly. You will notice that the side of this banana that rests on the counter will ripen more quickly than
the other sides, because it has the closest contact (接触) with its own ethylene. The banana that is tightly
wrapped in plastic has no oxygen supply and should ripen most slowly. Now can you see why bananas are
sealed in plastic in many supermarkets?

1. What makes the green banana in Bag 2 ripen more quickly than the ones in Bag 1?

A. Ethylene from the very ripe banana.
B. The free flow of oxygen or ethylene.
C. CO₂ from the very ripe banana.
D. The limitation of oxygen supply.

2. How can we slow down the ripening of bananas exposed to the air?

A. To rest them on the counter.
B. To keep them in the basket.
C. To put them on the ground.
D. To hang them up from the ceiling.

3. It can be concluded from the passage that _____.

A the ripening banana is a self-contained system
B. the rate of ripening bananas is affected by the environment
C. wrapped bananas experience no chemical changes to ripen
D. ripening bananas give off oxygen, CO₂and ethylene

4. Why are bananas sealed in plastic in many supermarkets?

A. To make them ripen more quickly.
B. To make them ripen more slowly.
C. To make banana flesh soften.
D. To turn green bananas yellow.