I would go for D but if in doubt go for B
In biology, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of residues that form temporary bonds with the substrate (binding site) and residues that catalyse a reaction of that substrate (catalytic site).
A) the aorta pumps blood out of the heart
The tissue that sends electrical signals through the body is the nervous tissue
