Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

In the early 1800s, less than 3% of the world's population lived in cities; today, more than half of the global population is urban and by 2050, the proportion will rise to three quarters. There are thousands of small and medium-sized cities along with more than 30 megacities and sprawling, networked metropolitan areas with 15 million residents or more. Yet despite these massive transformations in how people live and interact, our international affairs are still largely dictated by nation states, not cities.

Cities are beginning to flex their muscles on the international stage. They are already displacing nation states as the central nodes of the global economy, generating close to 80% of global GDP. Cities like New York and Tokyo are bigger in GDP terms than many G-20 countries. Metropolitan regions and special economic zones are linking global cities through transnational supply chains. A growing number of mega-regions, such as those linking cities in Mexico and the U.S., transcend borders. In the process, cities are collectively forging common regional plans, trading partnerships, and infrastructure corridors.

The spectacular rise of cities did not happen by accident. Cities channel creativity, connect human capital, and when well governed, they drive growth. That many cities and their residents are rolling up their sleeves and getting things done - where nations have failed - are grounds for optimism. In the future, we hope that it is our proximate, accountable, and empowered city leaders who will define our fates.

The phrase "flex their muscles" in paragraph 2 is closest in meaning to __________ .