As bananas ripen， the green skin turns ye11ow gradually. Chemical changes occur in the f1esh of the fruit as well：starch（淀粉）changes to sugar；pectin（果胶）breaks down，losing its stiffness； and the f1esh 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 fo1d 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 a1one or five days to ripen. Ripening fruit“breathes”． This means that it takes up oxygen and gives off C02. 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？
72. 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. C02 from the very ripe banana.                     D. The limitation of oxygen supply.
73. 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
74. 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, C0, and ethylene
75. 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.