In a comparison of birds and mammals, the condition of having four limbs is .
a. a shared ancestral character.
b. a character useful for distinguishing birds from mammals.
c. a character useful for sorting bird species.
d. a shared derived character.
e. an example of analogy rather than homolog

a. A shared ancestral character.

Explanation:

The condition of birds and mammals having four limbs is a shared ancestral character because the structure of the limbs are similar in both the cases.

The forelimbs of the bird are modified into wings to aid flying.The hind limbs are modified into feel and claws.

These are homologous structures to the hands and feet in mammals.

That means they have a common origin, and hence are shared ancestral character.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.