We are using LED bulbs for energy saving.

This cell is dividing very actively.

He received the letter challenging him to a duel and began preparing for the duel.

Citrate plays an important role in organic chemistry.

Gender roles are an important element in defining individual behavior in society.

His role is to fulfill his responsibilities as a leader.

We should save electricity for energy conservation.

We should increase the use of alternative energy to protect the earth.

The potential energy of this ball is large because it is in a high position.

The potential energy of this ball is proportional to its height.