a. All those who were overweight and not agile were exempted from participating.
b. All those who were overweight, and not agile, were exempted from participating.
Which should be used if the assumption is that none of those who were overweight were agile?
Which should be used if there were some overweight people who were agile and only those who were overweight and not agile were exempted?
Is (a) ambiguous?