It continued to increase.

Hi Ramin, 'It continued to increase.' is the best answer here. When you use a verb after 'continue' you can use infinitive with to OR verb+ing. So both these answers would be correct. 1. It continued to increase. 2. It continued increasing. There is no difference in meaning between these two sentences. For more information on verb patterns, have a look at this site: http://learnenglishteens.britishcouncil.org/grammar-vocabulary/grammar-videos/verb-ing-or-verb-infinitive I hope that helps. Christine