When Doing Gel Electrophoresis,why Do Researchers Add a "Ladder" Made

When doing gel electrophoresis,why do researchers add a "ladder" made up of molecules of known size?

A) to help the molecules they are studying get through the gel more easily
B) to help the researchers identify the size of each molecule after the separation has occurred
C) to make sure the radioactivity doesn't interfere with the movement of the molecules that are being studied
D) The molecules being studied would not show up without the ladder.