Ursula's used automobile dealership is suffering from a severe decline in the number of car sales. Ursula decides to test two approaches to sales. She randomly assigns her $16$ salespeople to two groups. One group is trained to use the "hard sell" approach and the other to use the "soft sell" approach so that she can determine which method is more effective. The number of sales for each group is compared before the study start and after the study ends. Identify the level of research interference:

Select one:

a$.$ Minimal

b$.$ Moderate

c$.$ Maximal

d$.$ Excessive