Research has shown that most economic profits from selling a prescription drug are eliminated 20 years after the drug is first offered for sale. The main reason for the elimination of profits is OA the quantity demanded of the drug has increased enough that the demand becomes inelastic and revenue falls OB. firms sell their patent rights to other firms so that they can concentrate on finding drugs to treat newitnesses. O c. after 20 years most people who have taken the drug have passed away or are cured of the lines the drug was intended to treat D. after 20 years patent protection is ended and other firms can produce less-expensive generic versions of the drug