What is the current status of research on memory-enhancing drugs?

What is the current status of research on memory-enhancing drugs?
 
  a. Mixed evidence for the effectiveness of the drugs in memory enhancement among normal individuals b. Strong evidence that the drugs enhance memory in normal individuals
  c. Strong evidence that the drugs enhance memory in normal individuals, but only if the drugs are taken for at least two years
  d. Strong evidence that the drugs enhance memory in normal individuals, but only if combined with memory- retraining exercises
  e. No compelling scientific evidence that the drugs enhance memory in normal individuals