Give the First Step for the Mechanism for Serine Protease

Give the first step for the mechanism for serine protease.

A) Histidine functions as a base ,increasing the nucleophilicity of serine,which attacks the carbonyl group.
B) The tetrahedral complex collapses,expelling the amino group.
C) Water attacks the acyl group of the acyl-enzyme intermediate.
D) The tetrahedral intermediate collapses,expelling serine.
E) The negatively charged oxygen hydrogen bonds with two peptide bonds.