[求助]prep一道数学题

The points A, B, C, and D are on a numer line, not necessarily in that order. If the distance between A and B is 18 and the distance between C and D is 8, what is the distance between B and D?

1) The distance between C and A is the same as the distance between C and B

2) A is to the left of D on the number line.

为什么答案会是E呢？

完全不明白。。。陷入困境~

请教各位大侠

恍然大悟。。。

THX~~

唉。。怎么感觉一碰到考试脑筋就揪成一团。。然后就不灵光了。。