All of the following factors can cause famines except A
the failure of food production to keep pace with population growth
B
distribution problems caused by political turmoil
C
new, higher-yield crops
D
crop failure brought on by sustained drought

C   new, higher-yield crops.

Explanation:

I'm certain that this is what they want you to say, but even this answer has caused famines. It comes down to whether the people will socially accept the newer high yield crop. Some people would rather starve themselves than consume an 'unacceptable' food.

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia