The limb bud in a developing human embryo at four weeks and the same limb bud at eight weeks are remarkably different. Which statement below explains how the changes in the limb bud are brought about?
A. Certain cells accelerate their rate of growth to produce elongated digits.
B. Some cells undergo a programmed death, allowing digits to emerge.
C. As cells differentiate, some migrate to different positions in the limb bud.
D. Cells not destined to become part of any digits migrate to other areas of the embryo.