Identification of the Rate-Limiting Step in the Synthesis of Norepinephrine $\beta$

Identification of the rate-limiting step in the synthesis of norepinephrine was achieved by measuring the

A) activity of dopamine $\beta$ -hydroxylase in the presence of varying levels of dopamine.
B) amount of DOPA produced after inhibition of tyrosine hydroxylase.
C) amount of each relevant enzyme in the cytoplasm.
D) amount of norepinephrine produced when the preparation was bathed in various precursors.
E) amount of norepinephrine produced when various enzymes were inhibited.