Usually the reason is D. It can make us biased, I don't see any other option making sense
Codominance is when both alleles are expressed - one allele is not more dominant than the other.
An example of this is a cow with red and white hair, you can see both colors of hair, not just one.
"Dad, it is just too hot to work in the garden." Dave grumbled as he hoed grass beside his father. He continued, "We could get a hot stroke from the sun."