土壤空气和大气进行交换的机制有（）和（），在一般情况下是以（）为主要交换方式。
参考答案：质流；扩散；气体扩散
---Why was Mr. Smith suddenly fired by his boss?
---Because he was found_______ in the non-smoking office.
A．smoking
B．smoke
C．to smoke
D．smoked
