27.

Animals cannot make arachidonic acid (20:4

5,8,11,14) de novo (from acetyl CoA) because they are unable to

a. insert double bonds between the methyl terminus and the C10 carbon.
b. insert double bonds between the carboxyl terminus and the C9 carbon.
c. extend the chain length beyond 18 carbons.
d. insert more than one double bond in each fatty acyl chain.