Birds and Mammals Have Very Different Typical Reproductive Strategies

Birds and mammals have very different typical reproductive strategies. What is a key reason for this?

A) In birds, both males and females can provide a great deal of parental investment for the young, because both can incubate the egg(s) and both can bring food to the chick(s) .
B) Birds are not as highly evolved as primates, so they have simple undifferentiated mating systems mostly involving male songs and calls.
C) With their larger brains, mammals can form a wider range of relationships than can birds and therefore can form more productive mating systems.
D) Because they can fly, birds do not experience limitations on food availability and therefore undergo mainly sexual selection and almost no natural selection.