$A$ microwave oven heats food containing water molecules very efficiently under which of the following conditions?

• A
  The frequency of microwaves has no relation to the natural frequency of water molecules.
• B
  Microwaves are heat waves, so they always generate heat.
• C
  Infrared waves in the microwave oven generate heat.
• D
  The frequency of microwaves must match the resonant frequency of water molecules.