In the business novel THE GOAL, when Alex tagged orders as red$/$green and required non$-$constraints to work on red tagged orders first, it is an example of primarily which of the five steps of focusing?

A$.$ Decide how to EXPLOIT the system's constraint$($s$).$

B$.$ SUBORDINATE everything else to the above decision.

C$.$ ELEVATE the system's constraint$($s$).$

D$.$ IDENTIFY the system constraint$($s$).$