In the 1800s, the united states was still a very young nation, trying to solidify its identity. the industrial revolution began in great britain, a fast development of society following the introduction of machines. the united states was slower than great britain to fully embrace the changes. yet key technological developments caused a rapid growth in american urban areas. better farming methods and tools in the 1800s increased food production. americans were able to grow enough food for their families as well as to sell. the abundance caused food prices to fall. the expansion of cotton and the growth of textile factories in northern states produce the first wave of american industry. more people turned to work in northern factories as a way to support their families. thousands of immigrants to the united states also settled in or near port cities, looking for work. even today, the need for work is a common reason people move to urban areas. as a result, cities grew in numbers of people and physical space. as more people and businesses moved in, they needed buildings for living and working. they needed ways to move around the city. we call this process urbanization. in 1820, the united states had only a few cities of 10,000 residents or more. about seven percent of u. s. residents lived in urban areas. the number of cities with more than 10,000 people grew quickly over the next 40 years, especially in the northeast and midwest. by 1860, about 20 percent lived in cities. philadelphia and new york city were the most populated cities in 1860 and would soon reach one million residents. the urbanization of the united states quickened due to technology improvements. without innovations in food production, the factories could not have grown so quickly. the trend quickened after 1860 and continued throughout the 21st century as well. by 2007, more americans lived in or near cities than they did in rural areas.

which sentence from the introduction paragraph serves as the author's thesis?

a. in the 1800s, the united states was still a very young nation, trying to solidify its identity.
b. the industrial revolution began in britain, a fast development of society following the introduction of machines.
c. the united states was slower than britain to fully embrace the changes.
d. yet key technological developments caused a rapid growth in american urban areas.