Can you help with this (i'm behind in math!)

Can you help Identify the sampling method used.?
At the 1-mile marker of a marathon, a timekeeper shouts out the time elapsed to every 10th runner that passes by. A statistician records the times shouted.
A. convenience
B. voluntary response
C. random
D. systematic

A geologist visits 10 randomly selected lakes in the region and collects soil samples in randomly selected areas along each shoreline.
A. systematic
B. convenience
C. random
D. stratified

d c