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Scientists have found that skin tissue is more resistant to toxic substances than lung tissue and uses sweat glands to protect the immune system from possible invaders.
skin tissue is more resistant to toxic substances than lung tissue and uses
skin tissue is more resistant to toxic substances than is lung tissue and uses
2nd option is correct one. what is wrong with the first one? aint first one suggests:"skin tissue is more resistant to toxic substances than lung tissue is"?
1. skin tissue is more resistant to toxic substances than lung tissue and uses
2. skin tissue is more resistant to toxic substances than is lung tissue and uses
or
3. skin tissue is more resistant to toxic substances than lung tissue is and uses...
Without the 'is', No. 1 could imply that skin tissue is less resistant to lung tissue, which does not make sense.
more ... than... parallelism 
Scientists have found that skin tissue is more resistant to toxic substances than lung tissue and uses sweat glands to protect the immune system from possible invaders. 
skin tissue is more resistant to toxic substances than lung tissue and uses 
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