Answer:
A. crowded and unsanitary apartment buildings where two-thirds of the city's population lived.
Step-by-step explanation:
In the 19th century, lots of people began crowding into America’s cities, including thousands of newly arrived immigrants seeking a better life than the one they had left behind.
In New York City; where the population doubled every decade from 1800 to 1880 buildings that had once been single-family dwellings were increasingly divided into multiple living spaces to accommodate this growing population. Known as tenements, these narrow, low-rise apartment buildings had lack services, which a normal apartment building would have. By 1900, some 2.3 million people (a full two-thirds of New York City’s population) were living in tenement housing.