If A, B, C, and D lie on the same straight line, and if AC = 2CD = 3BD, what is the value of the ratio BC/CD ?

The answer says/explains:

Let AC = 6; then CD = 3, with D on either side of C. BD=2, but B could be on either side of D, and so we have no way of knowing length BC. The value of the ratio BC/CD cannot be determined from the information given.

I don't understand how they came to that conlucsion.

AND why are there 2 D'S?! "D on either side of C"??