Answer: Bacteria
Explanation:
There are some things that could make people detest an activity, food, event or anything at all, and when this happens ones, they would rule off doing that activity or eating that food until they are convinced otherwise even though what happened wasn't a normal occurrence. Ying happens to be in this category of people as she detest eating salad sandwich due to her experience with it and bacteria sometime ago, these is known as unconditioned stimulus
