A
Dry ice is CO2 in solid form, with a temperature of about －78.5℃. Unlike common ice, dry ice doesn't melt into a liquid. When dry ice melts, it turns into a gas. So when you see dry ice “smoking”， it's frozen CO2 melting into its gaseous state. Dry ice is a popular matter in scientific experiments. It creates projects that look cool.
Experiment: Healthy Dry Ice Popsicles(棒冰)
This dry ice experiment will catch people's eye, especially little kids' because the dry ice is going to be used to make what the young love to eat. It should take you about 10 minutes or less to have a frozen treat to enjoy.
Materials
One large block of dry ice
A large spoon
Popsicle sticks
One cup of your favorite juice
A tall glass filled with water
Directions
Use the large spoon to make a popsicleshaped space in the block of dry ice. You can set the ice on a piece of cloth or plastic on your table to do this work. Of course, keep in mind that it's okay if it's not exactly popsicleshaped.
Add a popsicle stick to the middle of the space so that when you pour in the liquid, it will freeze around the popsicle stick.
Hold the popsicle stick in place and wait a few minutes for your popsicle to harden. After several minutes, take away the popsicle from the block of dry ice and put it into the tall glass of water for a few seconds. Do this to warm up the popsicle just a bit. Otherwise, it will be too cold.
本文为说明文，主要介绍了制作干冰棒冰的材料和过程。
21．What difference do dry ice and common ice have?
A．Their state after melting.
B．Their temperature.
C．Their usage.
D．Their form.
解析 A 细节理解题。根据第一段第二、三句Unlike common ice, dry ice doesn't melt into liquid…it turns into a gas可知，干冰不像普通的冰那样会融化成液体，因此它与普通冰的不同之处是融化后的形态，故选A项。
22．Which material in the following is not a must in making a popsicle?
A．Water. B．A spoon.
C．Sticks. D．Cloth.
解析 D 细节理解题。根据解析Materials下的内容可知，需要的材料包括干冰、勺子、棒冰棍、果汁、水，布不是必需品，故选D项。
23．What is a tall glass filled with water used for in the experiment?
A．To warm the popsicle. B．To harden the popsicle.
C．To cool the popsicle. D．To shape the popsicle.
解析 A 细节理解题。根据最后一段最后两句Do this to warm up the popsicle just a bit. Otherwise, it will be too cold可知，水的用途是给棒冰加温，故选A项。
