1.a. Ozone constitutes less than one part per million of the atmosphere. b. Ozone is extremely important for life on earth.

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two factors or characteristics of a chapter that might make it difficult to read would be words i don't understand, or if i don't fully understand what's going on in the chapter. strategies i can use to get through the difficult material would be a dictionary, and re-reading previous chapters to get a better grasp of what the author is trying to say.

there is no sentence to look at.