When Performing Gel Electrophoresis, Why Do Molecular Biologists Add a "Ladder

When performing gel electrophoresis, why do molecular biologists add a "ladder" made up of nucleic acids or proteins of known size?

A) To help determine the size of each macromolecule after the separation has occurred.
B) To help macromolecules move through the gel more rapidly.
C) To make sure the dye included in the sample buffer doesn't interfere with the movement of the macromolecules that are being studied.
D) The macromolecules being studied would be invisible without the ladder.