Complete the link sentence that leads onto the next paragraph which talks about 'natural gas': Electricity has become a very important part of our existence; we are extremely dependent on it. If there were no electricity, life as we know it would probably end. We feel helpless without electricity. On a hot day without electricity, we would feel frustrated. There can be no life without electricity, but ...

The 'topic sentence' introduces us to the subject of the paragraph and the 'link sentence' is a sentence that introduces the next paragraph: this ensures a smooth continuity in the text.