17. the answer is...

the Senate and House (of Representatives)

Congress is divided into two parts -the Senate and the House of Representatives. Because it has two "chambers," the U.S. Congress is known as a "bicameral" legislature. The system of checks and balances works in Congress. Specific powers are assigned to each of these chambers. For example, only the Senate has the power to reject a treaty signed by the president or a person the president chooses to serve on the Supreme Court. Only the House of Representatives has the power to introduce a bill that requires Americans to pay taxes.