A、An interesting story of a teacher. B、The advantage of a new invention. C、The way to make a useful pot. D、The way to keep food

A few degrees can make a big difference when it comes to food storage. Foods can go bad if they get too warm. But for many of the world’s poor, finding a good way to keep food cool is difficult. Refrigerators are costly and they need electricity. (33) Yet spoiled food not only creates health risks but also economic losses. Farmers lose money when they have to throw away products that they cannot sell quickly. But in 1995, a teacher in northern Nigeria named Mohammed Bah Abba found a solution. He developed the "Pot-in-Pot Preservation" or "Cooling System." It uses two round containers made of clay (泥土). A smaller pot is placed inside a larger one. The space between the two pots is filled with wet sand. The inner pot can be filled with fruit, vegetables or drinks. A wet cloth covers the whole cooling system. (34)Food in the smaller pot is kept from spoiling through a simple evaporation process. Water in the sand between the two pots evaporates through the surface of the larger pot, where drier outside air is moving. The evaporation process creates a drop in temperature of several degrees. This cools the inner pot and helps keep food safe from harmful bacteria. The invention became popular with farmers in other African countries. (35) Mohammed Bah Abba personally financed the first 5 000 pot-in-pot systems for his own community and five villages nearby. In 2000, the Rolex Watch Company of Switzerland honored him with the Rolex Award for Enterprise. This award recognizes people trying to develop projects aimed at improving human knowledge and well-being.
32. What does the speaker mainly talk about?
33. Why does the speaker say spoiled food can create economic losses?
34. How does the pot-in-pot system work?
35. What can we learn about Mohammed Bah Abba?