What is the $20-$minute rule?

Question $33$ options:

a$)$

Move to a different activity or learning method at least every $20-$minutes.

b$)$

Each of the five steps to teach a skill should last $20-$minutes each.

c$)$

Training modules should only last $20-$minutes.

d$)$

Set inductions should be no longer than $20-$minutes.