Part B
Directions: You will hear four dialogues or monologues. Before listening to each one, you will have 5 seconds to read each of the questions which accompany it. While listening, answer each question by choosing A, B, C or D. After listening, you will have 10 seconds to check your answer to each question. You will hear each piece ONLY ONCE.
听力原文： W: Did you hear the thunder last night？
M: Did I？ It was really loud.
W: I didn't see any lightning though. I thought that thunder and lightning always occur together.
M: They do. It's not possible to have one without the other since thunder is a direct result of lightning.
W: Really？
M: Yes. Thunder is produced by the rapid expansion and contraction of highly heated air along the path of an electrical discharge.
W: In other words, lightning quickly heats the air and causes thunder. But if the lightning is there, why can't we see it？
M: Because it sometimes occurs very high in the clouds.
W: I wonder whether it's possible to see the lightning and not to hear the thunder.
M: Yes. It's possible if the lightning flashes are far enough away. Sound doesn't travel as far as light.
What is the main topic of the conversation？

A. How lightning is produced.
B. The loudness of thunder.
C. The relationship between lightning and thunder.
D. How to determine the location of a lightning flash.