The principle of checks and balances refers to the system, in the American federal system, wherein each branch of government has responsibilities and obligations as it relates to the other branches of government.
The President, for one, is able to veto legislation from the Legislative Branch but a balance on that check is that the Legislative branch may override the Executive branch's veto.
Checks and balances is the system that gives each of the three branches of government ways to limit the power of other branches. Example; Executive branch, the legislative branch has the power to over ride vetos and impeach a president.
30S 90E = None of the above. It's in the Indian Ocean (They could have meant 30S 90W. That's in the Pacific Ocean, which is on the list but hasn't been used).
