A、The ways of keeping food eatable for a longer time. B、The killing of bacteria. C、The invention of the refrigerator. D、The role

In the old days, fresh food had to be eaten in the city no more than one or two days after it left the farm. If kept longer, it would spoil. One of the reasons behind it is that the air contains many living things that can harm our food. Some things, such as insects, can be seen by us. Others, like bacteria, can be seen only with a microscope.
    All living things need water and man discovered that if this water is removed, the activity of the bacteria which causes food to go bad is prevented. In other words, we can protect some of our food by drying it. Besides, living things can only grow at certain temperatures, so we can also protect our food by heating or cooling it.
    A New York man had an idea. He filled a wooden railroad car with tons of butter. The car was hooked to a train and pulled from New York to Boston. How could this be done? The butter was packed in ice, and twice during the trip more ice was added. This was the first refrigerator ear. As a matter of fact, People also used wooden boxes filled with ice to keep their food. Now we have mechanical refrigerator cars and mechanical refrigerators at home and stores and other places that need them.