For many centuries, the Chinese have used extracts from a group of herbs known as ephedra to

Question:

For many centuries, the Chinese have used extracts from a group of herbs known as ephedra to treat asthma. Chemists have been able to isolate a compound from these herbs, which they named ephedrine, a potent dilator of air passages in the lungs.

a. How many stereoisomers are possible for ephedrine?
b. The stereoisomer shown here is the one that is pharmacologically active. What is the configuration of each of the asymmetric carbons?