有道题看不懂。麻烦翻译一下。

The number line shown contains three points
R, S,
            and T, whose coordinates have absolute values
r, s, and t, respectively. 
Which of the following equals the average (arithmetic mean) of the
coordinates of the points R, S, and T ?

A.     s

B.     s + t - r

C.    

D.    

E.