表征岩层渗透性的指标主要是（）。
参考答案：渗透系数
He has been in bed ______ he fell off his bike a week ago.
A．even if
B．soon after
C．now that
D．ever since
保持指示空速不变，飞行高度升高，则真空速：（）.
A.大于指示空速
B.小于指示空速
C.保持不变
D.减小
