Final answer:
Young people contribute to the growth of towns and cities through rural-urban migration and high birth rates.
Step-by-step explanation:
The two missing words in the sentence are economic and natural.
Young people contribute to the growth of towns and cities by moving from the countryside to cities through rural-urban migration. This is an economic factor as it refers to individuals seeking better job opportunities and economic prospects in urban areas.
Young people also contribute to the growth of towns and cities by having lots of babies once they get there. This is a natural factor as it involves population growth through birth rates.
Learn more about Contribution of young people to urban growth