4.在真空中沿着z轴负方向传播的平面电磁波,电场强度波的方程为 _(y)=-|||--754cos [ omega (t-dfrac (pi )(u))+pi ] (SI), 则磁场强度波的方程为 ()-|||-(A) _(x)=2.0cos [ omega (t-dfrac (z)(u))+pi ] (SI)-|||-(B) _(x)=-2.0cos [ omega (t-dfrac (z)(u))+pi ] (SI)-|||-(C) _(x)=754cos [ omega (t-dfrac (z)(u))+pi ] (SI)-|||-(D) _(x)=2.0cos [ omega (t+dfrac (z)(u))+pi ] (SI)