Self-study
Concerns for the environment have led chemists to develop plastics that are degradable. ███ ██████████ █████████ █████ ███ ███████████ ██████ █████████ ██████████ ████ ████ ███ █████ ██████████ ██ █████ █████ ████ ████ ████████ ██ █████████ ████ ████ ██ ██ ██████ ██ ████ ███ ████ ████ ██ ██ █████████ ██ ██████ ██ ██████ ██ █████████ ████ ████ ██████████ ████████ █████ ████████ ██ ███████ █████████
Stimulus Breakdown
The stimulus tells us about the qualities of degradable plastics—some qualities that all degradable plastics have, and some qualities that only certain degradable plastics have. All degradable plastics are potentially useful packaging materials, and they all need specific conditions to break down. To break down, some degradable plastics must be exposed to sunlight, some must be buried in soil and some must be submerged in water. Finally, some degradable plastics leave residues of unknown toxicity.
One way to think about all these claims is as conditional statements:
(1) degradable plastic → potentially useful for packaging
(2) degradable plastic → specific conditions to break down
(3) degradable plastic ←some→ break downexposed to sunlight
(4) degradable plastic ←some→ break downburied in soil
(5) degradable plastic ←some→ break downsubmerged in water
(6) degradable plastic ←some→ unknown-toxicity residues
Another approach we can take is set logic: considering degradable plastics as a superset with certain qualities (potentially useful for packaging, needs specific conditions) that contains different subsets with their own qualities (breaks down in sunlight/soil/water, leaves residues).
Objective: Identify an Inference that Must Be True
Using either approach, we can start to think about interactions between these statements/sets that will allow us to make inferences. Specifically, any quality of the superset also belongs to the subsets: if all degradable plastics are potentially useful, that means degradable plastics which break down in sunlight are potentially useful, and so on. We can also state this as a set of new "some" statements: "some materials that are potentially useful for packaging break down in sunlight," and so on. Any statement of this kind, which is a logically valid inference, could be a correct answer.
In the answer choices, keep an eye out for invalid inferences that might try to trick us. For example, "nothing that is not a degradable plastic is potentially useful for packaging" and so on. If necessary, diagramming difficult answer choices is always an option, and can be useful to check against the claims in the stimulus.
5.
If all of the statements █████ ███ █████ █████ ███ ██ ███ █████████ ████ ████ ██ █████
a
Some materials that ███ ███████████ ██████ ███ █████████ █████ ████████ ██ ███████ █████████
This is a valid inference from statements (1) and (6) in the stimulus. All degradable plastics are potentially useful for packaging, and some degradable plastics leave unknown-toxicity residues, so it must be true that some materials which are potentially useful for packaging leave unknown-toxicity residues.
b
Some degradable plastics ████ ████ ████████ ███ ██████████ ██ █████ ██ █████ █████
Some degradable plastics need sunlight and some need soil—but we don't know if these sets overlap at all. This means we can't say if any degradable plastics need both. It could be true, but it doesn't have to be.
c
Some materials that ████ ████████ ██ █████ ██ █████ ████ ███ ███ ███████████ ██████ █████████ ██████████
We only know that some degradable plastics need sunlight; we don't know if any other material needs sunlight to break down. If we removed the "not," (C) would be a valid inference.
d
Some materials that █████ ████████ ██ ███████ ████████ ███ ███ ██████████ █████████
Exactly like (C), the "not" in (D) means it doesn't have to be true. We don't know about materials other than degradable plastics, so no statement about other materials has to be true.
e
Some materials that ████ ██ ██ ██████ ██ ████ ██ █████ ████ █████ ████████ ██ ███████ █████████
(E) makes the same error as (B): we don't know if these sets overlap. Some degradable plastics need soil and some leave residues, but it's entirely possible that those groups are fully separate. (E) could be true, but doesn't have to be.