The muscle layers in the stomach wall churn the food and mix it with the secretions from the stomach lining. These secretions are called gastric juice and contain a mixture of hydrochloric acid, mucus and a digestive enzyme pepsin. The acid makes the gastric juice overall very acidic, around pH 1.5.
The correct answer is fitness. The Theory of Evolution defines fitness as a result of the adaptations an organism has which increase its chance of survival.
