Question

why are we able to use nuclear energy for practical purposes?
the reactions are controlled to regulate energy output.
the reactions are uncontrolled for maximum energy output.
all of the products are fed back into the reaction to keep it going.
all of the products are immediately removed to inhibit more reactions.

Explanation:

Nuclear energy is used for practical purposes when the nuclear reactions (like nuclear fission in a nuclear reactor) are controlled. In a nuclear reactor, control rods (made of materials like cadmium) can absorb neutrons. By adjusting the position of these control rods, the rate of the nuclear chain - reaction can be regulated. This regulation allows for a steady and useful output of energy. If the reactions were uncontrolled (like in a nuclear bomb), it would be a violent explosion and not useful for practical, continuous energy production. If all products were fed back (without control), the reaction would be uncontrolled. If all products were immediately removed, the reaction would stop too quickly to be useful for energy production.

Answer:

The reactions are controlled to regulate energy output.