The correct options are
B Breastbones are modified to hold muscles of flight.
D Birds have hollow bones.
Birds have light and hollow bones. These bones allow them to lift themselves easily. The bony parts of the forelimbs are modified as wings. They have strong shoulder bones. Their breastbones are modified to hold muscles of the flight, which causes the wings to move up and down. Birds walk on the ground. So, their hindlimbs are modified for walking and perching.