在过量的空气中燃尽2.24L（标准状况）一氟代烃气体，燃烧后的生成物在27℃下全部为澄清石灰水所吸收，可得到13.9g沉淀物。求此气体的分子式。
CH3F
—Can you tell me ________ to send this postcard to New York by airmail?
—Let me check. Oh，it’s one dollar.
A．how long it takes
B．how much it costs
C．who can help me
D．if there’s a post office
如何开展会展评估工作？
