Whom Would You Trust as Most Reliable? Discuss the Credibility

Whom would you trust as most reliable? Discuss the credibility and authority of each individual or group listed with regard to the following issue:
A number of your friends have taken up jogging, and you wonder whether your taking it up might have genuine health benefits for you.
a. your family physician
b. a magazine for runners
c. a friend who teaches physical education in high school
d. the author of a best-selling book on sports medicine
e. a friend who is president of a local runners club