Consider the thyroid hormones, triiodothyronine (T3) and thyroxine (T4). What do the numbers "3" and "4" refer to?

a. The number of iodines found in each hormone.
b. The relative half-life of the hormones (3 or 4 hours).
c. None of the above.
d. The number of enzymes required for synthesis of each hormone.
e. The number of thyrosine molecules linked together to form the hormones.