The stimulus is a set of facts that is leading us to an obvious conclusion and it can be broken down as follows:
(1) Some cleaning fluids, synthetic carpets, wall paneling, and other products release toxins into the household air supply.
(2) Not a problem in ventilated houses but it is a problem in well-insulated houses.
(3) Recent tests demonstrate that houseplants remove some household toxins from the air.
Notice that these facts are leading us to the conclusion that houseplants can offset the negative effects of household toxins.
This is why (D) is the correct answer. The problem with (A) is that it is far too specific.
This stimulus is not just about benzene; it is about cleaning fluids, synthetic carpets, wall paneling and other products. Benzene is just used as one example.
Hope this helps! Please let us know if you have any other questions.
