In this task, you are given a question and answer options for that question. Using this information, you have to classify each text into different topics: medicine, nursery, psychology, chemistry, pharmacology, biology.
Q: Question: Enzymatic regulation by phosphorylation:
Options: <0> It is reversible. <1> It requires enzymes that directly introduce the Pi. <2> Introduce the Pi into acidic and basic amino acids. <3> Always activate enzymes. <4> It is carried out by phosphatases.
A:
biology