What is a secondary source? A. A source of historical information that is not written down
B. A source of historical information that has not been evaluated by
an expert
C. A source of historical information based on the interpretation of
other sources of historical evidence
D. A firsthand source of historical information​

D.A firsthand source of

i think the answer is d.

the more fit you are, the faster the recovery. your heart rate drops most sharply in the first minute after you stop exercising; it should then fall about 20 beats a minute—a drop of less than 12 beats a minute is considered abnormal. this “recovery heart rate” is measured as part of an exercise stress test.