Definitely take more time reading the stimulus! Make it a goal to understand and absorb every word in the stimulus, the question, and each answer choice.

The passage does not really give us detail about process of making garments. The only thing we can do is assume. Even so, it is not reasonable to assume that just because one part of the entire garment manufacturing process is cheaper, doesn't make the entire thing less expensive. What if the green and brown cotton, because they are so eco-friendly/look good, drive demand higher? What if the supply of green/brown cotton is less and businesses have to charge more? These are assumptions we cannot take and, if we picked those answers, would be left as holes in the argument-- ultimately making the argument not 100% valid.

The whole idea is that we are not sure if those other minerals besides ferrous oxide impacted the algae bloom. What if ferrous oxide is the only one that does that? There are too many assumptions to be made with that answer choice.