Starting the \parts environment with a different counter in exam class

Question

I need to be able to start a parts environment at a later counter. For instance, starting with part (b) instead of (a).

\documentclass{exam}
\begin{document}
\begin{parts}
\part this is part a.
\end{parts}

\begin{parts}
\part This should be part b. 
\part This should be part c. 
\end{parts}
\end{document}

I've tried the ways to do it with the list packages, but they don't agree with the exam class. I've done my best to comb through the exam class document but didn't find anything.

Answer 1

The exam class defines the partno counter, in order to achieve the startover at (b) the resetting of the partno counter has to be cheated by setcounter{partno}{1}.

\documentclass{exam}
\begin{document}
\begin{parts}
\part this is part a.
\end{parts}

\begin{parts}
\setcounter{partno}{1}
\part This should be part b. 
\part This should be part c. 
\end{parts}
\end{document}