Answer:the tough outermost membrane enveloping the brain and spinal cord
Explanation:
Alfred Marshal was one of the most influential economists of his time and the author of the Principles of Economics book which was the principal economic textbook in England for long years. During his economic theory development, he borrowed a concept from the marginalists which is the theory of demand and the collective result of buyers who yearns services and products or the concept of scarcity.
Graphite is well known to be pencil lead - You might actually learn this in Chemistry/Physical Science like we did!
Charcoal & coal you cannot use - you could but it'd be difficult and irritating.
Fullerene is the third form of carbon. It can be used for electronics AND medicine. Amazing right??