Oprea, E., & Underwood, C. (2016). The 4 x 4 diet: 4 key foods, 4-minute workouts, four weeks to the body you want (First edition.). Harmony Books.
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रOprea, Erin, और Carrie Underwood. The 4 X 4 Diet: 4 Key Foods, 4-minute Workouts, Four Weeks to the Body You Want. First edition. New York: Harmony Books, 2016.
एमएलए (8वां संस्करण) प्रशस्ति पत्रOprea, Erin, और Carrie Underwood. The 4 X 4 Diet: 4 Key Foods, 4-minute Workouts, Four Weeks to the Body You Want. First edition. Harmony Books, 2016.
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