9. The Fort Worth Zoo has several two-legged birds and four-legged mammals. During a visit to the zoo, Maggie counted a total of 200 heads and 522 legs among these animals. How many birds are there in the zoo? ( )
(A) 61
(B) 122
(C) 139
(D) 150
(E) 161
9. C.

Let the number of birds and mammals be $x$ and $y$ respectively. According to the problem,
$$
\left\{\begin{array} { l } 
{ x + y = 2 0 0 , } \\
{ 2 x + 4 y = 5 2 2 }
\end{array} \Rightarrow \left\{\begin{array}{l}
x=139, \\
y=61 .
\end{array}\right.\right.
$$