Why has it been necessary to make so many changes to our paper currency in the past 30 years as compared to the last 100 years

Counterfeit and Forgery.

Explanation:

Over the last 30 years changes have had to be made to the paper currencies due to high rate of counterfeit bills in circulation and forgery.

To detect a Counterfeit dollar bill, the falsified portrait is lifeless and flat. Details, often too dark, do not blend into the background. The Federal Reserve and Treasury seals are simple, distinct, and sharp on a genuine bill. There may be uneven, blunt, or broken on the counterfeit seals.

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d) center of the mid-ocean ridge

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