Applying Scientific Methods A student grew a yeast culture on sterilized nutrient medium in a closed dish for 5 days. Each day, she took the same size sample from the dish and placed it on a special slide used for counting microorganisms. She examined the samples under a microscope and drew the following illustrations of her observations over the course of the investigation. Each dot represents 10 yeast cells. Why did the student use sterilized medium and keep the dish closed?​