Scientist think this region may have once been a large sea plains

Have 10 people in each treatment complete the game.

Explanation:

When we do a scientific experiment, we create treatments to test the hypotheses that we established in the experiment. Each treatment will get the same number of repetitions, so that the evaluation of the treatments is done in an equal way providing a logical and correct result.

In the question above, we see that we are dealing with people playing while listening to soft music and using touch pad, people who play listening to soft music and using mouse, people who play listening to hard rock and using touch pad, people who play listening to hard rock and using mouse , and, people who are playing without listening to any music and without using a mouse or touch pad.

For the research evaluation to be done correctly and efficiently, it is necessary that each treatment has the same number of repetitions. That is, each treatment has the same number of people. Thus the correct thing to do would be to have 10 people in each treatment to complete the game.