A、The robes must be made from cotton. B、The robes must be in white. C、The robes must be loose. D、The robes must be in black. C细节

M: Hey, Yael,(13)check out my new white suit. It’s going to keep me cool on my vacation to the Mojave desert.
W: That is one modern suit, Don.
M: Oh, I’m styling. Plus, everyone knows that white reflects heat and black absorbs it. Yes, if you’re out in the sun, you’re better off wearing white.
W:(13)Not always, Don. After all, Bedouins, the nomadic people who spend their entire lives in the desert, wear black robes.
M: But that doesn’t make sense. Dark surfaces get warmer in the sun than light surfaces. You’d think the Bedouins would have figured that out by now.
W: Don, Don, Don. Things are never that simple. You’re right that the air underneath black fabric warms up faster than the air underneath white fabric.(14)At the same time, though, black fabric provides more shade than white fabric, and this decreases the amount of light that directly reaches the skin. Plus, a lot depends on the type of clothing you’re wearing. You see, warm air rises. And when it does, it’s replaced by cool air. And if you happen to be wearing a robe, all that movement of the warm air creates a breeze that sucks up cooler air from the bottom of the robe and pushes it out the top.
M: So wearing a black robe is like having a suit with a built-in fan.
W: Exactly. But again,(15)the key is that the robe is loose-fitting. Otherwise, there isn’t enough room for the air to circulate. So if you’re going to the desert in a tight-fitting suit, it’s better to wear white.
M: My suit is white.
Questions 13 to 15 are based on the conversation you have just heard.
13. What is the practical reason for people live in dessert wearing black robes?
14. What of the following is true?
15. What is key for those people living in dessert wearing robes?