丙烯汽化器引丙烯时要（）。
A、先引丙烯，后投蒸汽
B、先投蒸汽，后引丙烯
C、丙烯、蒸汽一起投用
D、无需投用
参考答案：B
—Could you tell me what time the train leaves Hefei for Beijing?
—I'm afraid you need to _____ on the Internet. [ ]
A. look for it
B. look at it
C. look it over
D. look it up
青霉素可产生
A．后遗反应B．停药反应C．特异质反应D．变态反应E．副反应
