In order for natural selection to have produced the long necks of giraffes, which of the following must have been true over the evolutionary history of giraffes?
a. In every generation, the length of giraffes' necks showed variation.
b. Abundant resources insured that competition among giraffes was limited.
c. The length of necks in giraffe offspring was always longer than both parents.
d. All giraffes, regardless of neck length, had an equal chance of survival.