We would like pD=0 when p=pi ， pD to increase with time and pD to be positive.So we rewrite pD in the form（）.

We would like pD=0 when p=pi ， pD to increase with time and pD to be positive.So we rewrite pD in the form（）.
A.pD=（pc-p）/pi
B.pD=（p-pc）/pi
C.pD=（p-pi）/pc
D.pD=（pi-p）/pc
正确答案：pD=（pi-p）/pc