During the Late 1800s,cattle Egrets Arrived in South America from Africa

During the late 1800s,cattle egrets arrived in South America from Africa and began to colonize.Their range has expanded dramatically over the years.They were able to do this because

A) the habitats that they left in Africa were not suitable for any further colonization;thus,they were forced to immigrate.
B) the habitats that they encountered in South America were suitable to them and unoccupied.
C) there were abundant cattle for the birds to gather around in South America;furthermore,various animals that the egrets had lived around in Africa had become quite scarce because of over hunting and poaching,causing the birds to extend their range.
D) the food resources in South America were far superior to those in Africa,allowing the egrets more opportunity to grow and reproduce and ultimately expand their range.