In the mechanism of chymotrypsin,which of the following amino acids found in the active site is correctly defined in terms of its role in the reaction?

A)serine: hydrogen bonds with carbonyl oxygen to withdraw electron density from the substrate
B)histidine: deprotonates aspartic acid to allow a nucleophilic attack to occur
C)aspartic acid: electrostatic stabilization of histidine to make a stronger base
D)cysteine: performs a nucleophilic attack on the carbonyl carbon of the substrate
E)none of the above