28. As shown in Figure 6, a, b, c, $d, e, f, g, h, i$ represent one of the numbers $1,2,3,4,5,6,7,8,9$ respectively. By adding 3 numbers in 9 circles, we get 9 sums. If these 9 sums are equal, then the value of $a+d+g$ is ( ).
(A) 15
(B) 16
(C) 18
(D) 19
(E) 21
28.C.

Let this common sum be $s$. Adding these 9 sums, we get
$$
\begin{array}{l}
9 s=1+2+\cdots+9+2(a+b+\cdots+i) \\
=3 \times 45=9 \times 15 .
\end{array}
$$

Thus, $s=15$. The sum of the numbers in the 6 circles is
$$
\begin{array}{l}
90=6 s \\
=1+2+4+5+7+8+(a+b+\cdots+i)+a+d+g \\
=27+45+a+d+g=72+a+d+g .
\end{array}
$$

Therefore, $a+d+g=90-72=18$.