question: 28 years old female on oral contraceptives was given tetracycline for treatment of  chronic periodontitis by a dentist. After 2 months she became pregnant. What is the  reason of contraceptive failure:
opa: Tetracycline induces metabolism of OCD
opb: Tetracycline inhibits enterohepatic circulation causing increased excretion of OCD
opc: Tetracycline inhibits metabolism of OCD
opd: Tetracycline displaces OCD from plasma protein