Write the Correct Letter in the Table Below (Some May

Write the correct letter in the table below (some may be used more than once and some not at all).
Compare and Contrast:
Discrete Trial vs. Free Operant Procedures
$\begin{array}{l}&Discrete ~Trial&Free~ Operant\\\hline\text{Is there an}S^{D}\text{ and an }S^{?}&-&-\\\hline\text{Is there an intertrial interval}&-&-\\\hline\text{The measure is}&-&-\\\hline\end{array}$
a. yes
b. no
c. sometimes
d. usually not
e. latency or accuracy
f. rate
g. volume
h. intensity