What is the basic difference between the classical design and the Solomon four$-$group design?

The Solomon four$-$group design has randomization.

There is no difference.

The Solomon four$-$group design repeats the classical design but adds groups that are not pretested.

The Solomon four$-$group design repeats the classical design but adds groups that are not posttested.

More time elapses between the stimulus and the second observation in the Solomon four$-$group design.