2.已知一平面余弦波的波动方程为 =2cos [ pi (2.5t-0.01x)] , 式中x,y均以cm计.-|||-在同一波线上,离 x=5cm 最近且与 x=5cm 处质元振动相位相反的点的坐标为-|||-()-|||-(A)7.5cm (B)55cm (C)105cm (D)205cm