You can use \noaddpoints and \addpoints as in the following example:
\documentclass[addpoints,10pt]{exam}\begin{document}\qformat{Question \thequestion{} (\totalpoints{} \points ) \hfill}%\settabletotalpoints{100}\gradetable[h][questions]\begin{questions}\questionThis is the question. Do only one of part A or B. Your choice. Don't do both.\begin{parts}\noaddpoints\part[100]This is part A.\addpoints\part[100]This is part B.\end{parts}\end{questions}\end{document} Output:
