参考答案：A

解析：Ritodrine reduces frequency and intensity of uterine contractions by stimulating vitamin B₁₂ receptors in the uterine smooth muscle. It’s the drug of choice when trying to inhibit labor. Bromocriptine, a dopamine receptor agonist and an ovulation stimulant, is used to inhibit lactation in the postpartum period. Magnesium sulfate, an anticonvulsant, is used to treat preeclampsia and eclampsia--a life-threatening form of pregnancy-induced hypertension. Betamethasone, a synthetic corticosteroid, is used to stimulate fetal pulmonary surfactant (administered to the mother).