A、By its length. B、By its taste. C、By its shape. D、By its color. D

When people think of carrots, they usually picture in their mind a vegetable that is long, thin and orange. But carrots come in many different sizes and shapes. And not all carrots are orange. For example, Paris Market carrots are about 5 centimeters around. Imperator carrots are thin and about 25 centimeters long. And Belgian White carrots are, as their name suggests, white. Weather, soil conditions and age will affect the way carrots taste. According to experts, warm days, cool nights and a medium soil temperature are the best conditions for growing carrots that taste great. Carrots need time to develop their full sugar content. This gives them their taste. If they are harvested too early, they will not have enough sugar. But carrots lose their sweetness if you wait too long to pull them from the ground. The best way to judge if a carrot is ready to be harvested is by its color. Usually, the brighter the color is, the better the taste. Many people do not know that carrots can be grown during winter months. If the winter is not cold enough to freeze the ground, you can grow and harvest carrots the same way as during summer months. If the ground does freeze in your part of the world, simply cover your carrot garden with a thick layer of leaves or straw.
Questions 19 to 21 are based on the passage you have just heard.
19. What may happen if we wait too long to pull carrots from the ground?
20. How can people know if the carrot is ready to be harvested?
21. What do many people NOT know about carrots?