It's been a really hot, humid day, and Karthik has spent it running his first marathon. After he finishes, he reports to the medical staff that he has chills. They notice, too, that his skin is so cool that he's got "goose bumps." Without knowing anything more about Karthik's condition, which of the following does he likely have?
A) heat cramps
B) sunstroke
C) heat exhaustion
D) heatstroke