Limb bones within unrelated animals that have the same basic structures are considered

a. homologous structures.
b. analogous structures.
c. embryological.
d. vestigial structures.

Limb bones within unrelated animals that have the same basic structures are considered homologous structures. Homologous structures are similar structures shared by unrelated species. An example is that birds and humans have the similar anatomy of limb bones, however, these are two unrelated groups of animals that share similar structures. Therefore, this is the example of  A. homologous structures.
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The answer is a because - is an direction