The critical mass of a fissile assembly can be influenced by several design modifications:
I. Surrounding the assembly with a thick neutron reflector.
II. Mixing the fissile material with a strong neutron absorber.
III. Introducing a moderator that slows fast neutrons to thermal energies.
Which combination of the above changes will effectively decrease the critical mass?
a) I only
b) I and III
c) II only
d) II and III