28. As shown in the figure, in $\triangle A B C$, if $A B=\frac{\sqrt{7-A C}+\sqrt{3 A C-21}+2 A C+4}{2}$, then the range of the median $A D$ on side $B C$ is ( ).
A. $2<A D<16$
B. $0<A D<16$
C. $1<A D<8$
D. $3<A D<8$
Answer: C