特别广播英语第 75 课：You Have a Choice 你可以选定一种解释-1

Two trains are traveling side by side and at the same speed along parallel tracks. 两列火车以相同的车速并排地沿着两条平行的轨道往前行驰。

We are seated in one of the trains, and with us we have a special speedometer that measures their relative speed. 我们坐在其中的一列火车上带着特制的速度测量仪，可以测出两列火车之间的相对速度。

Since the trains are traveling at the same speed, their relative speed is zero; the speedometer therefore reads "0." 因为这两列火车是以相同的车速前进的，所以二者的相对速度是零；因此，测速仪上的读数为"0"。

Suddenly the other train seems to start pulling ahead of ours. 突然间另一列火车使我们觉得好像是开始赶到我们所乘坐的火车前头去了。

The speedometer shows a reading of 10 miles per hour. 这时测速仪上显示出来的读数是每小时10英里。

The other train has apparently increased its speed. But can we be absolutely certain of this increase? 很显然，另一列火车已经加快了速度。但是，我们能不能肯定是另外那辆火车加快了速度呢？

If your answer is yes, you are wrong. You are wrong because all that we know is that the relative speed between the two trains changed from 0 mph to 10 mph. 如果你回答说是的，那你就错了。你之所以是错了，我们所知道的仅仅是这两辆火车之间的相对速度由每小时0英里增加到了每小时10英里。

Nothing more. This change could have been brought about in one of two ways: 仅此而已，这车速的变化很可能是由于下列两种原因中的任何一个原因所造成的：

The other train increased its speed. 另一列火车增加了车速；

Our train decreased its speed. 我们的火车降低了车速。