$36 \cdot 29$ There are 50 girls, whose skin is either white or light black, and their eyes are either blue or brown. If 14 have blue eyes and white skin, 31 have light black skin, and 18 have brown eyes, then the number of girls with brown eyes and light black skin is
(A) 5 .
(B) 7 .
(C) 9 .
(D) 11 .
(E) 13 .
(22nd American High School Mathematics Examination, 1971)
［Solution］, there are $50-31=19$ with white skin. $\therefore 19-14=5$ have brown eyes and white skin. $\therefore 18-5=13$ have brown eyes and light black skin. Therefore, the answer is $(E)$.