It is illegal for a public official to ask people for gifts or money          favors to them.

A．in preference to

B．in place of

C．in agreement with

D．in exchange for

用化学知识解释下列问题：

（1）有人说发明了“水变成汽油”的技术，请说明这是伪科学的理由．

（2）蜡烛燃烧后质量变小了，请说明理由．