为什么在停止乙烯进料后,需要用己烷冲洗管线?
参考答案:
防止堵塞。
As the years passed, many occasions—birthdays, awards, graduations — __________ with Dad’s flowers.
A.are marked
B.were marked
C.have marked
D.had marked
列车编组顺序表记事拦记事用代号中,限速连挂车辆用()表示。
A、X
B、XL
C、U
D、XG