Nhận xét nào sau đây là đúng? A: Không khí là hỗn hợp của nhiều chất trong đó chủ yếu là khí oxi, khí nitơ... B: Không khí chỉ bị ô nhiễm ở một vùng, không lây lan ra nơi khác. C: Không khí là hợp chất của hai nguyên tố là oxi và nitơ. D: Không khí chứa oxi nhiều hơn nitơ.

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