某段距离的平均值为100mm,其往返较差为+20mm,则相对误差为( )。

A、<img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/153560949977259c700812c9c4eeab9814465db057c7cimage1.png">;

B、<img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/153560949988723b5d68b73354a1d99cf82481eccb3eaimage2.png">;

C、<img src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/examination/imgUplod/153560949997963b85811e90d43d2ab8bad3d3b614a27image3.png">