Pharmaceutical companies are selling billions of dollars' worth of single-enantiomer drugs. Why might it be important to use only one form of an enantiomer?

• A. It is cheaper to manufacture only one of the enantiomers.
• B. It is not important. Both enantiomers will be equally effective.
• C. The left-handed version of the enantiomer is always the effective one.
• D. One enantiomer may provide an effective treatment whereas the other may be ineffective or even toxic.