The PK_aʹs of the Side Chain Group and the α-Carboxyl

The pK_aʹs of the side chain group and the α-carboxyl group of glutamate are 4.1 and 2.1, respectively. Which statement accounts for this difference?

A) The side chain has more possible resonance structures.
B) The α-carboxyl group has less steric hindrance and is therefore ionized more easily.
C) The side chain is a different functional group than the α-carboxyl group.
D) The α-carboxyl group is closer to the α-amino group than the side chain is.