The correct option is this: A STRONG WIND BLOW ACROSS A MIDWESTERN PRAIRIE.
A strong wind is an example of a natural force. The prairie refers to a land fill with grasses and that is a natural factor also. Thus, a strong wind that is blowing across a prairie is generating a natural form of energy. The energy flow in the other options given are examples of man made devices.
I believe that would be small polar and nonpolar molecules.
Hope it helps!
Photsynthesis to make food
