Four positive integers are given. Select any three of these 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: 
$\textbf{(A)}\ 19 \qquad \textbf{(B)}\ 21 \qquad \textbf{(C)}\ 23 \qquad \textbf{(D)}\ 29 \qquad \textbf{(E)}\ 17$
Define numbers $a, b, c,$ and $d$ to be the four numbers. In order to satisfy the following conditions, the system of equation should be constructed. (It doesn't matter which variable is which.)
\[\frac{a+b+c}{3}+d=29\]
\[\frac{a+b+d}{3}+c=23\]
\[\frac{a+c+d}{3}+b=21\]
\[\frac{b+c+d}{3}+a=17\]
Adding all of the equations together, we get: $2(a+b+c+d)=90$. This means that $a+b+c+d=45$.
We can determine that $\frac{a+b+c}{3}+d+16=a+b+c+d$. This, with some algebra, means that $\frac{1}{3}(a+b+c)=8$. $d$ must be $\boxed{\textbf{(B)} 21}$.