26) A

The texts says that only female birds have the medullary bone. So in order to prove that a fossil is male, then it is necessary for it not have the medullary bone.

However, the text also says that the medullary bone can deplete in females, meaning that if a fossil without a medullary bone is found, then it can either be male or female.

So it is a necessary but not a sufficient condition.

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