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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.