Chapter 6.1, Problem 20E

Problems 21-30 refer to the table below of the six basic solutions to the e-system

$\begin{array}{l}2x_1+3x_2+s_1=24\\ 4x_1+3x_2+s_2=36\\ \begin{array}{ccccc}& x_1& x_2& s_1& s_2\\ \left(\text{A}\right)& 0& 0& 24& 36\\ \left(\text{B}\right)& 0& 8& 0& 12\\ \left(\text{C}\right)& 0& 12& -12& 0\\ \left(\text{D}\right)& 12& 0& 0& -12\\ \left(\text{E}\right)& 9& 0& 6& 0\\ \left(\text{F}\right)& 6& 4& 0& 0\end{array}\end{array}$

In basic solution $\left(\text{D}\right)$, which variables are basic?