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A developing human embryo produces a special type of hemoglobin that is not found in the blood of an infant, child, or adult.
Which statement best explains how this can be?

a) Humans have genes for different types of hemoglobin, and they are expressed at different times.
b) Human genes change slowly as a person ages.
c) An embryo and an adult have different genes for hemoglobin.
d) As the embryo develops and cells differentiate, certain genes are lost.