--- Betty , welcome to my birthday party . Please make yourself at home .

--- ________.

A．The same to you

B．No , not really

C．Thank you

D．Here you are

杆重力为P，重心在C，弹簧弹性系数为k，当杆处于铅直位量时，弹簧无变形，已知杆对θ的转动惯量为J，则杆绕O点做微小摆动时的运动微分方程为（）。

A.A

B.B

C.C

D.D