Which of the following best characterizes the pattern of ossifcation (hardening) that occurs in a (long) bone?
a. Crossification starts on one end and continues to the other
b. ossification starts in the middle of the shaft of the bone and spread to either end
c. ossification starts from the outside of the bone (along the periosteum) and continues inward
d. ossification starts first in the middle of the shaft and then on either end of the bone