Answer:
Enzymes are biological catalysts. They speed up reactions although they are not changed in the reaction. Enzymes are proteins, and therefore are folded chains of amino acids with a specific shape. This shape is determined by the sequence of amino acids held together by bonds, for example Hydrogen bonds.
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.