A、It tastes better. B、It is easier to grow. C、It may be sold at a higher price. D、It can better survive extreme weathers. C短文中提到

Agricultural workers in green tea fields near Mount Kenya are gathering the tea leaves. It is beautiful to see. The rows of tea bushes are straight. All appears to be well. But the farmers who planted the bushes are worried.
    Nelson Kibara is one of them. He has been growing tea in the Kerugoya area for 40 years.(19-1)He says the prices this year have been so low that he has made almost no profit. He says he must grow different kinds of tea if he is to survive.
    Mr. Kibara and hundreds of other farmers have been removing some of their tea bushes and planting a new kind of tea developed by the Tea Research Foundation of Kenya. Its leaves are purple and brown. When the tea is boiled, the drink has a purple color.
     (19-2/20)Medical researchers have studied the health benefits of the new tea. They say it is healthier than green tea and could be sold for a price that is three to four times higher than the price of green tea.
    But Mr. Kibara says he has not received a higher price for his purple tea crop.(21)He says the market for the tea is unstable. And he is often forced to sell his purple tea for the same price as green tea leaves. He says there are not enough buyers willing to pay more for the purple tea.
Questions 19 to 21 are based on the passage you have just heard.
19. Why have tea farmers in Kenya decided to grow purple tea?
20. What do researchers say about purple tea?
21. What does Mr. Kibara find about purple tea?