High-intensity training involves performing resistance exercises a. for a duration of at least 45 minutes. b. to the point of temporary muscle failure. c. without machines or equipment. d. until you start to feel minor muscle and joint pain

The answer would be D. until you start to feel minor muscle and joint pain. This is design to increase muscle strength thus, when muscle tearing occurs, it is believed that a stronger muscle will develop, thus the pain. With that an increasing resistance training is spurred for further improvements.