The condition under which a microwave oven heats up a food item containing water molecules most efficiently is:

1. A
   The frequency of the microwaves must match the resonant frequency of water molecules.
2. B
   The frequency of the microwaves has no relation with natural frequency of water molecules.
3. C
   Microwaves are heat waves, so always produce heating.
4. D
   Infra-red waves produce heating in a microwave oven.