It is difficult to measure the fitness costs of inbreeding because the frequency of inbreeding is usually low. However, Brown and Brown (1998), during a long-term study of the Mexican jay (Aphelocoma ultramarina), found that

A) fewer of the inbred nestlings survived to the next year when compared with outbred nestlings.
B) inbred pairs appeared to experience a higher rate of hatching failure.
C) the brood sizes of inbred pairs were smaller than those of outbred pairs.
D) all of the above.