一个盒中有m个红球，7个白球，n个黑球，每个球除颜色外其他都相同．从中任意取出一个球，取得的球是白球的概率与不是白球的概率相同，则m与n的关系是什么？

When Steven suddenly remembered that he had left his mobile in the office, the train was already ________.

A．on the go

B．on the rise

C．on the run

D．on the move