Answer:
When an action gets increasingly common.
Explanation:
The past progressive verb form illustrates an action that was in continuity or progress in the past. It displays the several actions like actions of the past that were interrupted, the two actions happened simultaneously, an action taking place when another happened but it would best describe the action when it gets increasingly common. For example, "He was drafting a letter when I reached there" or 'They were cooking dinner and I was watching T.V. effortlessly'. These two statements best illustrate the increasingly ongoing actions of the past that are "writing", "cooking", "watching" which were in progression at that time.
