The structure marked A begins to close by what time frame and due to what cause?
Begins to close at 10-15 hours after bih, due to withdrawal of prostaglandins Begins to close at 10-15 hours after bih, due to expression of prostaglandins Begins to close 4 weeks after bih, due to fall in oxygen concentration Begins to close 4 weeks after bih, due to rise in oxygen tension