<p>1) Obviously you are because each sentence has only 1 error.</p>
<p>2) Yes, there is no other way to refer to the river (eg, you can’t say " . . that moutnain range is the source of a river which course was not fully mapped until this century.")</p>
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<p>Yes. Remember, if you don’t include the phrase that you’re suggesting, the sentence would be saying that the use of plastics is greater than steel istelf, rather than the use of or that of steel, which is a very important distinction. Good of you to point that out by the way, the SAT loves that type of sentence error. :)</p>