A、Place it at the top of the cooler. B、Place it at the bottom of the cooler. C、Put its ends in the water. D、Put it outdoors. C细节

Before refrigerators were invented, the next best thing was an ice box. But another way to keep foods fresh is to use an evaporative cooler. A common design is a tall box with several shelves inside to hold the foods. The shelves are pieces of metal with many small holes through them. The sides of the box are covered with pieces of thick cloth. Containers of water are placed at the top and bottom of the cooler. The ends of each piece of cloth lie in the water so the cloth stays wet. The cooler is put outdoors, but not in the sun. Air will pass through the wet cloth. The inside of the box will stay several degrees cooler than the outside air temperature. And this may be cool enough to keep foods fresh, at least for a short time. Some foods can make you sick if they are stored in conditions that are not cold enough to prevent the growth of harmful organisms. Freezing can keep some foods in good condition for months after the growing season. Yet foods can be damaged if they are kept too cold. The British development group Practical Action says the best way to prepare foods for storage is at harvest time while still in the field. Use a sharp knife and place the harvested items on a clean surface or directly into storage containers. Do not put them on the ground.
16. How do people keep foods fresh before refrigerators were invented?
17. What should be done to keep the cloth wet?
18. What’s the best way to prepare foods for storage according to the passage?