【单选题】

{{I}}Questions 17-19 are based on the passage you’ve just hearD、{{/I}} What is the reason for calling the U.S. a"melting pot"

A、Its population consists of people of various nationalities.
B．The people tend to gather in racial groups.
C．It has always kept its door open to immigrants.
D．People all over the world have been to the country.