What structure most directly stimulates a skeletal muscle fiber to contract?

How does the closing of the ductus arteriosus and ductus venosus alter circulation in an infant? a. increases blood flow to the pulmonary artery and increases blood flow to the umbilical vein b. increases blood flow to the pulmonary artery and reduces blood flow to the umbilical vein c. reduces blood flow to the pulmonary vein and increases blood flow to the umbilical artery d. reduces blood flow to the pulmonary vein and reduces blood flow to the umbilical artery