What can be concluded about the distance from point b to cd?

Remark
Good thing you tried to answer the question. At lease we know how to proceed. 

A is wrong, because you can draw a conclusion.

B Remember that CD is constructed so that AB is cut in half. So the distance from CD to B is the same distance as CD to A.  Choice B is true.

C is incorrect. CD can be extended to any length as long as CD goes through the point where the two arcs above and below AB meet are joined by C and D, then CD can be any length. In order words, CD can be any length.

D Doesn't have to be. See the answer for C.