Which of the following steps in the folic acid synthesis pathway is specifically inhibited by sulfonamides?

A) the conversion of tetrahydrofolic acid to PABA
B) the conversion of PABA to dihydrofolic acid
C) the conversion of dihydrofolic acid to tetrahydrofolic acid
D) the conversion of PABA to tetrahydrofolic acid
E) the conversion of dihydrofolic acid to PABA