Fill in the Letter for Each of the Four Empty

Fill in the letter for each of the four empty cells:
Differential-Reinforcement Procedure vs.
Stimulus Discrimination Procedure
$\begin{array}{|c|c|c|}\hline & \begin{array}{c}\text { One } \\\text { Stimulus }\end{array} & \begin{array}{c}\text { Two } \\\text { Stimuli }\end{array} \\\hline \text { One Response Class } & \# 1 & \# 2 \\\hline \text { Two Response Classes } & \# 3 & \# 4 \\\hline\end{array}$
- combined differentiation and discrimination