[单项选择]一个人以6米/秒的速度慢跑一段路程，然后以4米/秒的速度返回到原处，那么这个人的平均速度是( )。
A. 5米/秒
B. 4米/秒
C. .4.5米/秒
D. 4.8米/秒

听下面一段对话，回答下列问题。
 

听下面一段对话，回答下列问题。

听下面一段对话，回答下列问题。
W: Hi, Bill, can you tell me how it happened
M: Sure. I was mountain climbing in New Hampshire in 1982. Suddenly the weather became really bad. There was a lot of snow and we couldn’t see anything. We got lost. Well, we spent four days on the mountain. The temperature was 20 degrees below zero, We didn’t have any equipment or food.
W: So what happened I guess someone found you, right
M: Yes, but we were very sick. I couldn’t move my legs because of the cold. f spent two months in the hospital. The doctors removed my legs.
W: Right. So you lost your legs, but you want to try your best to stay active.
M: That’s right. In fact, I decided to make some new legs for myself. I realized that no one had to be physically disabled. We can use modern technology to help us.
W: And you built these great new legs. Can you go mountai
A. Design new climbing shoes.
B. Establish a club for the disabled.
C. Use technology to fight his disability.

听下面一段对话，回答下列问题。
 

听下面一段对话，回答下列问题。
W: Hi, Bill, can you tell me how it happened
M: Sure. I was mountain climbing in New Hampshire in 1982. Suddenly the weather became really bad. There was a lot of snow and we couldn’t see anything. We got lost. Well, we spent four days on the mountain. The temperature was 20 degrees below zero, We didn’t have any equipment or food.
W: So what happened I guess someone found you, right
M: Yes, but we were very sick. I couldn’t move my legs because of the cold. f spent two months in the hospital. The doctors removed my legs.
W: Right. So you lost your legs, but you want to try your best to stay active.
M: That’s right. In fact, I decided to make some new legs for myself. I realized that no one had to be physically disabled. We can use modern technology to help us.
W: And you built these great new legs. Can you go mountain climbing again
M:
A. Low temperature.
B. Terrible weather.
C. An unsuccessful operation.