20. A class is fully occupied by students with no empty seats, and all desks are designed for two people. Among them, $60 \%$ of the male students have male classmates as their desk partners, while $20 \%$ of the female students have female classmates as their desk partners. Therefore, the percentage of female students in the entire class is ( )\%.
(A) $13 \frac{1}{3}$.
(B) $23 \frac{1}{3}$.
(C) $33 \frac{1}{3}$.
(D) $43 \frac{1}{3}$.
Answer: $\mathrm{C}$.
Solution: Let there be $m$ boys and $n$ girls in the class.
Since
the number of boys sitting with girls is equal to the number of girls sitting with boys,

i.e.,
$$
(100 \%-60 \%) m=(100 \%-20 \%) n \text {. }
$$

Thus,
$$
m=2 n \text {. }
$$

Therefore, the proportion of girls among all students is
$$
\frac{n}{m+n} \times 100 \%=\frac{n}{2 n+n} \times 100 \%=33 \frac{1}{3} \% .
$$

Hence, the correct choice is C.