What is the recommended frequency, intensity, and time to improve cardiorespiratory endurance?

A. 3-5 days/week, 51-65% max heart rate, 45 minutes/workout

B. 3-5 days/week, 51-65% max heart rate, 60 minutes/ workout

C. 5-7 days/week, 51-65% max heart rate, 45 minutes/workout

D. 5-7 days/week, 66-85% max heart rate, 60+ minutes/workout