A、When they’re unable to control the behavior. B、When the causes of the behavior are obvious. C、When the behavior may cause unpl

M: One of the most common questions we ask about people’s behavior is "why". Why she says this? Why he does that? Sometimes the reason is obvious, for example, someone is driving down the street, the light turns red, they stop, why?
W: Because they have to, legally, I mean.
M: Exactly! In this case the reason is obvious, so we usually don’t question it.[20]But when the reason is not so obvious and especially when the behavior could have negative consequences, we’ll more likely feel a need to explain the causes of the behavior.
W: Yes, maybe.
M: Social psychologists usually consent that there are two categories of reasons for people’s behavior: internal factors and external factors.
W: Professor, I think I understand a little, but could you explain it in detail?
M: Alice, say you’re driving down the road and all of a sudden some guy turn into the lane right in front of you, and you have to slam on your brake to avoid an accident. How do you react?
W: I’ll probably get very angry.
M: Why?
W: Well, he’s not paying attention. He’s a bad driver.
M: So you automatically attribute the driver’s behavior to an internal factor. He himself is to blame because he is careless.
W: So if I said it was because of heavy traffic or something, I’d be attributing his behavior to an external factor, something beyond his control.
M: Good. Now how do we usually explain our own negative behavior?
W: We blame external factors.
M: That’s right.
19. What are the two speakers talking about?
20. When are people more likely to try to explain the causes of the behavior according to the man?
21. How do most people explain their own negative behavior?