14. if you can't take your heart rate, a good way to recognize that you're exercising efficiently is whether or not

a. you're experiencing a little bit of huffing and puffing."> a. you're experiencing a little bit of huffing and puffing. b. you're gasping for breath."> b. you're gasping for breath. c. your muscles are burning."> c. your muscles are burning. d. it's difficult to have a conversation."> d. it's difficult to have a conversation.