Help me with some SATwriting problems pls~

Test Preparation SAT Preparation
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  1. (Along) the edge of the trail (stands) the huge boulder (that make) the canyon (so unnerving) to traverse.

The correct answer is B but I chose C. I think C should be “that makes”…

  1. The Pacific Northwest (averaging 4000 wildfires a year, caused mostly) by lighting. B. averages 4000 wildfires a year; most are caused

Why B not A?

2
  1. C is the answer for sure, unless you've mis-typed the sentence.
  2. It can't be (A) because "averaging" is a participle, not a verb, and thus makes (A) a fragment. Like this: Bob running a marathon. See how that's different from "Bob is running a marathon" or "Bob runs a marathon"?
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Thanks so much for helping me~

4

Gimme a “like” or “helpful” or sth, yo. Lame, I know, but those little things motivate me a bit to take time out of my (very busy) day to help out on here.