This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following is a 'nootropic' drug 
Select and explain. 
A. Rivastigmine, B. Tacrine, C. Amantadine, D. Piracetam
D: Piracetam
Answer: D: Piracetam 
Explanation: Nootropic drugs are those that enhance memory. Piracetam is used for this function in patients with head injury.