A、Chemicals. B、Vapour. C、Water. D、Gas. D细节题。短文开头明确提到，干冰是通过将气体直接冷冻形成的(byfreezing a gas)，可见干冰是用gas制成的，故答案为D)。

Can you imagine ice that does not melt and is not wet? Have you ever heard of dry ice?[22]Dry ice is made by freezing a gas. It is quite different from ordinary ice which is simply frozen water.[25]Dry ice was first manufactured in 1925. It has since fulfilled the hope of its inventor.[23]It can be used for making artificial fog in the movies. When steam is passed over dry ice, a very dense vapour rises. It can also be used for destroying insects in grain supplies.[24]It is more practical than ordinary ice because it takes up less space and it is 142 degrees colder. Since it turns into steam, instead of melting into water, it is cleaner to use. For these reasons, it is extremely popular and many people prefer it to ordinary ice. Dry ice is so cold that if you touch it with your bare fingers, it will burn you.
22. What is dry ice made of?
23. How is an artificial fog made in the movies?
24. What is the advantage of dry ice over ordinary ice?
25. When was dry ice first made?