A、The shape of the dust particles in the air. B、The relative humidity. C、The temperature of the air. D、The geography of the area

W: Have you ever looked really closely at the snowflakes?
M: Sure, but they usually melt too fast for me to get a close look. Why do you ask?
W:[12][13]I’m just curious. I was reading an article about the formation of snowflakes, and I realized that I had never paid much attention to them before.
M: Well, there is a big variety, isn’t there?
W: Yeah, but they all have one of the three basic forms: hexagonal columns, thin hexagonal plates and the branching star-shaped form.
M: I wonder why the forms are different, maybe because ice starts to form on dust particles with different shapes.
W: Well, I thought it might have something to do with the water saturation of the air. But we’re both wrong.[14] The author of this article did extensive research and concluded that the shape of snow crystals is largely controlled by the temperature of the air. For example, the feathery star-shaped snowflake that everyone thinks is typical occurs only at a specific temperature.
M: Doesn’t the relative humidity have anything to do with the shapes?
W: Apparently not. The effect of super saturation is simply to alter the growth rate. The greater the saturation, the faster the snowflakes form.
M: Wow,[15]next time it snows, I’ll make a point of taking a close look.
12. What are the two persons talking about?
13. Why has the woman brought up the subject of snowflakes?
14. What determines the shape of the snowflakes?
15. What does the man say he’s going to do?