The letters A, B, C, D, E, F and G, not necessarily in any order, stand for the digits 1 to 7. The following are known:
a. D is 4 less than A.
b. B is the middle term.
c. A and B are consecutive digits, but we don’t know which is bigger.
d. E and C lie between B and D.
e. G is greater than F.

What is B - D?
a) 1
b) 2
c) 3
d) 4