主汽门、调速汽门严密性试验时，试验汽压不应低于额定汽压的（）。
A、80%；
B、70%；
C、60%；
D、50%。
参考答案：D
There has been_____ rising demand for grain as_____ industrial material in spite of the nation’s
good harvests.
A．/, an
B．a, an
C．the, an
D．a, /
Helping others is a habit，______you can learn even at an early age.
A it B. that C. what D. one
