A is greater than B
C is less than D
I've made a table listing all the possibilities.
 A          B           C          D
 9          3           4           5/1
 6          2           3           4/0
 3          1           2           3/-1
The last column under D can be a little confusing but it is just a list of values taking into consideration the fact that D can either be either greater or lesser than B since the question stated difference. But by finding C, we can see that D has to be greater than B, otherwise, the values of C would be either 0 or negative.
From this, we can clearly see that B is the smallest.