Most Nuclear Reactors in the United States Are Thermal Fission

Most nuclear reactors in the United States are thermal fission reactors, reactors that slow their fission neutrons by sending those neutrons through materials known as moderators. Water is a common moderator. When a fast moving neutron enters water, it usually slows down because it

A) drags the water molecules with it and experiences water resistance.
B) pushes and pulls on the electrons in the water molecules and transfers most of its energy to those electrons.
C) creates bubbles in the water as it passes and those bubbles carry away most of its energy and momentum.
D) collides with the water nuclei and transfers most of its energy and momentum to those nuclei.