1. Which sentence best states the central idea of the text?
A. Millions upon millions of passenger pigeons once flew in flocks over the
North American continent.
B. The passenger pigeon's behavioral habit of living in huge flocks will
prevent it from being brought back to fly on Earth again.
C. It took humans and their technology to kill billions of passenger pigeons in
order to use them for food.
D. Humans caused the extinction of the passenger pigeon, but scientists are
attempting to bring it back through cloning.