Which contains a heavy atom often used in nuclear fission reactions to produce thermal energy?
uranium
carbon
petroleum
hydrogen

The correct answer is "uranium".

Explanation:

The nuclear energy is originated from the splitting of uranium atom in the nuclear fission.  

Nuclear fission is a process in which the bigger nucleus splits into the daughter nuclei with huge amount of energy. Uranium contains a heavy atom often used in nuclear fission reactions to produce thermal energy.

Uranium-fuelled in nuclear power plant is an efficient way of boiling water to make steam.There is an immense amount of heat produced in the nuclear reactor. Then, steam will produce. This steam turns turbines to generate electricity.

Therefore, the correct option is uranium.

Cross-pollination

Explanation:

cross-pollination is when pollen from one plant gets transported to another plant.

self-pollination is when pollen gets transported from the anther to the stigma of the same flower or a different flower on the same plant.