You seem to misunderstand the box model - in CSS you provide points for the top and left and then width and height - these are all that are needed for a box to be placed with exact measurements.
The width
property is what your C-D
is, but it is also what A-B
is. If you omit it, the div will not have a defined width and the width will be defined by its contents.
Update (following the comments on the question:
Add a border-bottom-style: none;
to your CSS to remove this style from the bottom only.