If you want to improve your muscular endurance, what is the best plan?

low resistance with high reps
high resistance with low reps
low resistance with low reps
high resistance with high reps

Endurance would be something that you could do for a long time, but not a lot of weight/pressure coming with it. So if you were to run, and wanted to do it for a while, Would you want low resistance and high reps or high resistance and low reps?  Obviously you would want something easier so you could keep a pace and do for a longer period of time. My answer would be A.