Imagine sailing through the sky, high above buildings and trees, in perfect peace and quiet. No noisy engines. No window with dirty marks in your way. No strong wind blowing in your face. You can't feel or hear the wind because you're traveling in the exact same direction and at the exact same speed it is. A hot-air balloon has three main parts: the balloon, called an envelope; a basket, where passengers ride; and a burner system, which supplies the hot air. Most balloon envelopes are made of strong, lightweight nylon.  They vary in size and shape, but they have to be really, really big to lift even a single person. Most weigh more than an adult man and use more fabric than you'd need to cover a baseball diamond . The basket is usually made of wicker ,which is strong but not too stiff . It needs to bend a bit for a soft landing, just like a good jumper bends her legs instead of keeping them stiff and straight when she lands. When the burner system blasts  flames, it makes a noisy sound, but once the balloon is flying , the burner can be turned off. The air in the envelope will then slowly cool, and the balloon will gradually sink unless the pilot starts the burner again. To operate, a pilot adds hot air to the balloon to go up or deflates it to go down, until he finds a breeze blowing in the direction he wants to fly. But no matter which way the wind blows them, balloonists have an amazing bird's-eye view of the world. The envelope of a hot-air balloon is used to   _  .
supply the hot air hold the hot air change the height control the direction
Question: Imagine sailing through the sky, high above buildings and trees, in perfect peace and quiet. No noisy engines. No window with dirty marks in your way. No strong wind blowing in your face. You can't feel or hear the wind because you're traveling in the exact same direction and at the exact same speed it is. A hot-air balloon has three main parts: the balloon, called an envelope; a basket, where passengers ride; and a burner system, which supplies the hot air. Most balloon envelopes are made of strong, lightweight nylon.  They vary in size and shape, but they have to be really, really big to lift even a single person. Most weigh more than an adult man and use more fabric than you'd need to cover a baseball diamond . The basket is usually made of wicker ,which is strong but not too stiff . It needs to bend a bit for a soft landing, just like a good jumper bends her legs instead of keeping them stiff and straight when she lands. When the burner system blasts  flames, it makes a noisy sound, but once the balloon is flying , the burner can be turned off. The air in the envelope will then slowly cool, and the balloon will gradually sink unless the pilot starts the burner again. To operate, a pilot adds hot air to the balloon to go up or deflates it to go down, until he finds a breeze blowing in the direction he wants to fly. But no matter which way the wind blows them, balloonists have an amazing bird's-eye view of the world. The envelope of a hot-air balloon is used to   _  .
A. supply the hot air
B. hold the hot air
C. change the height
D. control the direction
Answer: