When Glaxo-Wellcome Introduced AZT,an AIDS Drug,it Was Able to Enjoy

When Glaxo-Wellcome introduced AZT,an AIDS drug,it was able to enjoy high profits because:

A) it was highly recommended by the doctors.
B) the quick response of rivals in introducing substitute drugs.
C) barriers to entry provided by patents.
D) of its competitive price in the pharmaceutical industry.
E) it experienced constant returns to scale in the long run.