More specifically, production of thunder involves heating and cooling air quickly. When lightning occurs, it instantly heats the air around it. The hot air expands quickly. After the lightning disappears, the air cools and contracts. This process of expansion and contraction produces sound waves that we hear as thunder.

a. Thunder is a specific result of hot air meeting cold air quickly.

b. To be precise, thunder is caused by air expanding and contracting.
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More specifically = To be precise

expansion and contraction = heating and cooling

저는 답이 b라고 생각하는데요 다른 쌤이 a라고 이미 가르쳤더라구요..단순히 더운 공기와 찬공기가 만나는 차원이 아니라 그로인한 팽창과 냉각이라는 더 깊은 뜻이 있기 때문에 답이 b가 되는 거 맞나요??