I'll have to beg off . I have another engagement.恐怕我去不了了. 我另有个约会.We were invited to stay for dinner, but we had beg off.他邀请我们吃晚饭, 他这麽做真是太客气了.I'll try to beg off for an hour.我将设法请一小时的假....
我们可用反应式表示这种化学变化.This chemical change can be expressed by an equation.按照反应式生成烷基离子.Formation of an alkyl ion occurs according to reactions.这个化学反应的反应式是刚才所讨论的那个反应式的逆转.The equation for this chemical reaction is the reverse of...