Being interrupted before you've finished is an unpleasant feeling! The unpleasant feeling profoundly stayed in the memory of the people in group B, and could not be forgotten for a long time. Group A, on the other hand, completed the task normally, because the "desire to complete" was satisfied, they quickly forget the task. This effect is called the "Cecconi effect (Zeigernic effect)". The Cicconi effect, also known as the Zeigernic memory effect, refers to the fact that people are naturally driven to finish what they start and finish what they finish. It is easy for people to forget what they have already accomplished because their motivation to finish has been satisfied. But for those who have not yet completed the work can give them a very deep impression, so that they always want to go to the thing finished.
The Cecconi effect can lead people to two extremes: the unfinished complex, where they can't let go of what's left unfinished. That's why first love without results is so hard to forget. The second is that things are only done at the beginning, often halfway, never completely finish a thing.