There are two ways for animals to digest food, intracellular digestion and extracellular digestion. Intracellular digestion is a single-celled protozoan that swallows food particles into cells for digestion. Which of the following belong to intracellular digestion? ['The stomachs of birds digest food.', 'Intravenous glucose solution when you cannot eat when you are sick.', 'Cows ruminate the food they eat at rest.', 'Paramecium swallows Bacillus subtilis living in fresh water.']