问一个最新prep上面的数学问题

lwying

R,S,T and U are points on a line, and U is the midpoint of line segment ST. If the lengths of line segment RS,RT and ST are 20,4,and 24,respectively, what is the length of line segment RU?
A.6
B.8
C.12
D.14
E.16

差点儿要事业

直接上图啦，比较直观！