Read the following context and choose the best option to answer the question.
Context: You Ca n't Fool The Heart Rate MonitorA corollary to the expression , " You can fool some of the people all of he time , etc . " is that " you can never fool the heart rate monitor".This past Friday and Sunday , I did difficult 40 mile rides with my chavrusah . Trying to keep up with him was quite challenging but the results were excellent . I finished the ride in a personal best time and my average heart rate for the ride was around 80 % of my maximum heart rate .
Question: Why is the narrator 's heart rate so elevated ?
Options: 
A. They just did a bike ride .
B. They just did a marathon .
C. They just did a stressful work event .
D. None of the above choices .


A