Which of these is a complex sentence?

Immigrants who came to work in factories lived in urban neighborhoods.

"Old" immigrants often settled on farms, but most "new" ones lived in cities.

In the past, many immigrants had come from northern and western Europe.

Immigrants working long hours and facing crowded living conditions.

What is the columbian exchange? o a. the changes that resulted from europeans settling in the new world b. the relationships that columbus and his men had with the natives they met o c. the ships that columbus bought with money from the king and queen of spain o d. the increase in the number of native american towns in the new world