井中测量的视瑞利波的波速以（）为上限，以（）为下限。
参考答案：地层横波速度15千英尺/秒；井内流体速度9千英尺/秒
To our surprise, we got ______ people to attend the meeting as we expected.
A．as twice
B．twice many
C．twice many as
D．twice as many
快速成型技术主要有哪几种？简述其基本原理。
