A:—Who is she?—She's Tomy.
B:—Who is he?—He's Jim.
如图,P﹣ABCD是正四棱锥,ABCD﹣A1B1C1D1是正方体,其中AB=2,PA=6,则B1到平面PAD的距离为____
下列环境问题与防治措施说法合理的是( )