Which of the following statements about the genetic code is true?
a. a codon can specify more than one amino acid.
b. every codon specifies a different amino acid.
c. some codons specify the same amino acid.
d. some codons have no function at all.

c. Some codons specify the same amino acid.

Explanation:

Codons are triplets of nucleotides in mRNA that is used for the protein synthesis (translation). A codon specifies a single amino acid, but there are exceptions. tRNA molecule contain anticodons, triplets of nucleotides that are complementary to codons. So, during the translation, tRNA carries the amino acid, that corresponds to  the codon in mRNA.

you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).