A West Coast University Statistics Professor Wishes to Determine Which

A West Coast university statistics professor wishes to determine which surfboard brand delivers consistently longer rides, Channel Island or JS. He recruits two of his students who surf to assist in an experiment, and the student assigned to the Channel Island board is determined by coin flip. The three head out to the beach for an afternoon, with the professor holding up a sign at random times for the respective student to catch the next wave on his board, and the time spent on that wave is recorded. This is repeated until each student has ridden 10 waves.
The brand of board they're riding on is not the only factor affecting the length of time they ride each wave. It is likely that the two surfing students are not identical in surfing ability. Since the professor is only interested in determining which surfboard brand delivers longer rides, the study suffers from:

A) confounding.
B) voluntary response.
C) convenience sampling.
D) invalid measurement.
E) the placebo effect.