To remove the warnings,
Font shape `OT1/cmss/m/n' in size <4> not available(Font) size <5> substituted Size substitutions with differences(Font) up to 1.0pt have occurred.
I used the package lmodern, as stated here. However, it does create some problems in figure and sub-figure captions.
MWE:
\documentclass{article}
\usepackage{lmodern, subfigure}
\begin{document}
\begin{figure}
\subfigure[$A$-−$B$-−$C$-−$D$-−$A$-−$C$]{
}
\caption{$A$-−$B$-−$C$-−$D$-−$A$-−$C$}
\end{figure}
\end{document}
Screenshot:

How to fix that?
−is not a hyphen, it is a n-dash which is in TeX notation--. Delete all those-−and replace them with---– Jun 10 '14 at 06:21