There is a "blood-brain barrier" between the human brain and the blood. Any substance that acts as a sleeper must first pass through this barrier to work. Milk contains an amino acid called tryptophan that can cross the blood-brain barrier. It makes serotonin, a hormone that induces sleep, so drinking milk before bedtime is considered a very effective way to promote sleep. Which of the following, if true, would most weaken the conclusion? ['When the skin temperature rises, you will fall asleep faster, so drinking a cup of hot milk is like taking a hot bath to speed up falling asleep.', "The small amount of tryptophan contained in a small portion of milk is not enough to cause large fluctuations in the body's hormone levels. Only a large amount of milk can help sleep.", 'Carbohydrates such as rice are better at helping sleep. They stimulate insulin synthesis and allow amino acids other than tryptophan to enter muscle tissue, making it easier for tryptophan to enter the brain.', 'Milk contains many types of amino acids. When these substances enter the blood, they will compete for passage through the blood-brain barrier, thereby reducing the ability of tryptophan to cross the blood-brain barrier.']