Why the part question labels are not vertically left aligned in exam class

Question

I'm using TexLive 2015 under Windows XP, and using the latest exam class. Here is the sample code build by Xelatex engine.

\documentclass[a4paper]{exam}

\begin{document}
\begin{questions}
\question[10] questions
\begin{parts}
\part[2] part one
\part[4] part two
\part[4] part three
\end{parts}
\begin{solution}[10cm]
solution
\end{solution}

\end{questions}
\end{document}

The result screen shot see below.

You see that the part (b) and (c) are not vertically aligned. Any ideas? Thanks.

EDIT: as Manuel told me that it did aligned at the ), so my question is: Is it possible to both aligned at ( and ), only aligned at ) dose not look quite nice as the screen shot shows. Because (c) looks a bit smaller than the (b) as shown in the image.

EDIT2: I mark egreg's answer as the solution, since I think align on both ( and ) is not a good idea, see comments below.

Answer 1

You can use (perhaps abuse) the \partshook command, that's usually empty, in order to modify the definition \parts does for \makelabel; \parts defines \makelabel to do \hss\llap{##1}, which makes for the label to be right aligned in the reserved space, so we can make it do \rlap{##1}\hss (that is, the reverse).

\documentclass[a4paper]{exam}

\renewcommand{\partshook}{%
  \renewcommand\makelabel[1]{\rlap{##1}\hss}%
}

\begin{document}
\begin{questions}
\question[10] questions
\begin{parts}
\part[2] part one
\part[4] part two
\part[4] part three
\end{parts}
\begin{solution}[10cm]
solution
\end{solution}

\end{questions}
\end{document}