<s>[INST] context: Production of ketone bodies by the livera. As glucagon levels rise, adipose tissue breaks down its triacylglycerol stores into fatty acids and glycerol, which are released into the blood.b. Through the process of b-oxidation, the liver converts the fatty acids to acetyl coenzyme A (acetyl CoA).c. Acetyl CoA is used by the liver for the synthesis of the ketone bodies, acetoacetate and b-hydroxybutyrate. The liver cannot oxidize ketone bodies and hence releases them into the blood.Blood glucose is the major fuel for the brain, except when, after 3 to 5 days of fasting, ketone bodies in the blood reach a concentration at which the brain can begin to utilize them. Even at this point the nervous system still obtains 60% of its energy from glucose. The brain does not take up, to an appreciable extent, fatty acids, and fatty acid oxidation does not provide much energy for the brain. The brain's glycogen stores are very small, and alanine is not used as an energy source under any conditions by the nervous system. [/INST] question: Choose the major fuel that is being used by the brain after 1 day of fasting. | options: A: Ketone bodies B: Blood glucose C: Fatty acids D: Glycogen | answer: B </s>