they told you what you need to know - A, B, C, D are on a line, but not in this order.
Stat. (1) suggests this configuration: C is the midpoint between A and B, with a distance of 18/2=9 either way.
A----C----B
We know that C to D is 8, but we don't know in which direction: D could be to the left (AD---C----B) or right (A----C---DB) of C, and the distance from B to D (requested by the question stem) will be different in each of these scenarios. Stat. (2) mandates that A be to the left of D, but since CD is only 8 while AC is 18/2=9, this means that A is to the left of D in both the above configurations - so stat. (2) does not eliminate one of the scenarios, and there are still two possible values for the distance BD. Thus, combination is insufficient, and the answer is E.
