How does the U.S. economy create and destroy millions of jobs each year?

How does the U.S. economy create and destroy millions of jobs each year? (Mark all that apply.)

A. When unemployment is high, the federal government hires more workers to offset the effects of unemployment.
B. Some businesses are expanding, while others are contracting.
C. The federal government determines that certain industries are obsolete and closes firms.
D. In the market system, new firms are constantly entering and exiting various industries.