In the ¹H NMR spectrum of mechlorethamine, which of the following statements most accurately describes the relationship between the chemical shift of proton environment A and proton environment B?
a) Proton environment A is more shielded than proton environment B.
b) Proton environment A is less shielded than proton environment B.
c) Proton environment A is more deshielded than proton environment B.
d) Proton environment A is less deshielded than proton environment B.