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<p>The correct sentence: In the United States, the industrial use of plastics is greater than the use of steel, aluminum, and copper combined.</p>
<p>Or: In the United States, the industrial use of plastics is greater than THAT of steel, aluminum, and copper combined. </p>
<p>The error in the original is an illogical comparison. The sentence compares the USE of something to metals. You would need to compare the USE of something to the USE of something else.</p>
<p>And for your 3rd question, although that’s a relatively easy one that I would have gotten right, someone else will probably be able to explain a little better than I can.</p>