Sentence Correction

I'm not posting any question, but had a doubt while tackling a particular question.
Any help will be greatly appreciated.

While they remove carbon dioxide from the air, conserve soil and water, and house thousands of species, forests also supply potentially valuable pharmaceuticals, and, as sources of building material and firewood, that provide employment for millions worldwide.
a)pharmaceuticals, and, as sources of building material and firewood, that provide
b)pharmaceuticals and, as sources of building material and firewood, provide

I had a doubt regarding the use of and in the above 2 sentences. Can anyone explain.
As far as my understanding goes, the option b) uses and to create parallelism.
what is the role of and in b?
really confused with comma , and, comma ?

Thanks,
~Akshat