I have a hard time with this question. Please help.

Question

One way to determine the concentration of protein in a solution is to perform a "protein assay". There are several assay kits on the market that one can use, but they all rely on the common paradigm that the aromatic amino acid residues of a protein combine with the reagents from the protein assay kit to form a colored solution within 30 minutes. Using a spectrophotometer, one compares the intensity of the color of one's unknown protein solution with the intensities of color of the standards of known concentrations. Interestingly enough, you have a solution containing an unknown concentration of a polypeptide with the sequence "MetCysAlaArgLysLeuThrGlnVal". You perform the protein assay and no color forms. You wait and wait and still no color forms. Color does not form even after several hours because.

a. The peptide is too small to cause a color reaction
b. There are no amino acids with aromatic rings in their structure
c. You forgot to turn on the spectrophotometer
d. Your standard curve had a "R" value of 0.899

I believe the answer is "A" but am not sure. Please help

Which amino acids have aromatic rings? I think you are guessing.