Commas

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00I was wondering if these sentences are right? I think I placed the commas in the right place.02br
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001) When Mount St. Helens erupted in 1980, the north slope collapsed, sending torrents of mud and rock down into the Toutle River valley.02br
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002) Stripped of all vegetation for fifteen miles, the valley was left virtually lifeless; whatever tress there were, were dead.02br
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003) In an effort to prevent erosion and speed, the valley’s recovery ecologists planted grasses and ground covers.02br
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004) However, the species they planted were not native, but alien or exotic.030src/Themes/default/images/spacer.gif