Which is not and example of the system of checks and balances at work?
a. the president vetoes a bill.
b. a supreme court justice retires.
c. the senate refuses to approve a treaty.
d. the federal courts block an executive order.

B. A Supreme Court justice retires.

Explanation:

    The whole system can be exemplified by the process of processing a bill. For a law to be passed, it takes a majority of votes in both Legislative Houses - House and Senate. “If Congress legislated alone, it could become a sovereign organ, it would be the most influential organ of all. To prevent the power of Congress from being excessive, there is the veto figure. The executive branch can veto laws that come from the legislative branch. This is a classic formula for checks and balances.