20. Jeff went to the store to buy socks, which are priced at 1 yuan, 3 yuan, and 4 yuan per pair. If Jeff paid a total of 24 yuan for 12 pairs of socks, and he bought at least one pair of each price, then how many pairs of 1-yuan socks did he buy?
(A) 4
(B) 5
(C) 6
(D) 7
(E) 8
20. D.

Let Jeff buy $x$ pairs of socks at 1 yuan per pair and $y$ pairs of socks at 3 yuan per pair. Then
$$
\begin{array}{l}
\left\{\begin{array}{l}
x+3 y+4(12-x-y)=24 \\
x, y \geqslant 1, \\
12-x-y \geqslant 1
\end{array}\right. \\
\Rightarrow 6 \frac{1}{2} \leqslant x \leqslant 7 \frac{2}{3} \text {. } \\
\end{array}
$$

Therefore, he bought 7 pairs of socks at 1 yuan per pair.