Four positive integers are given. Select any three of the integers, find their arithmetic average, and add

Question:

Four positive integers are given. Select any three of the integers, find their arithmetic average, and add this result to the fourth integer. Thus the numbers 29, 23, 21, and 17 are obtained. One of the original integers is:
(a) 19
(b) 21
(c) 23
(d) 29
(e) 17