波动压力
参考答案：
由于钻具在井内流体中上下运动而引起井底压力减少或增加的压力值。是激动压力和抽吸压力的总称。
Mary is one of the girls who _____ always on time.[ ]
A. is
B. were
C. are
D. was
论述儿歌的主要特点并举例说明。
