阳毒内消散、红灵丹，功能活血止痛、消肿化痰，适用于一切阳证；阴毒内消散、桂麝散、黑退消，有温经活血、破坚化痰、散风逐寒之功，适用于一切阴证。
参考答案：对
I don't know _____ I should buy a present. [ ]
A. what
B. if
C. which
D. as
简述自然排烟的优缺点？
