We know that flooding is only possible in layer2 switch. But in OSPF router in layer3 uses flooding with multicasting. Suppose we have one router in OSPF uses multicast address 225.0.0.9. If router sends a message, every router get the message belongs to same group using class D address 225.0.0.9. But my questions are

A) If I using multicast address so why flooding is necessary because by multicasting everyone get message?

B) If I use unicast instead of multicast what's problem will be happen ?