Many important types of medicine have been developed from substances

The conclusion of this argument is: If the tropical rain forests are not preserved, important types of medicine will never be developed.

Here's how the author gets to this conclusion:

• Many important types of medicine have been developed from substances found in plants -- specifically, plants that only grow in tropical rainforests.
• Thousands of plant species in these rain forests have not yet been studied by scientists.
• It's very likely that many of those plant species contain substances of medicinal value.
• So if the tropical rainforests are not preserved, then important types of medicine (made from substances inside tropical rain forest plants) will never be developed.

We're asked if choice (D) is an assumption required by the argument. Let's take a close look at (D) to decide:


"Any substance" here has the same meaning as "every substance" or "all substances." So according to choice (D), if any substance at all has medicinal value, then that substance will eventually be discovered. In other words, every single substance of medicinal value will be discovered.

But does the conclusion really assume that every single substance of medicinal value will be discovered upon study?

No. Scientists do not have to discover every single substance of medicinal value in order to develop important types of medicine. Put another way, scientists can develop important types of medicine after studying fewer than 100% of the plants that contain these substances. Nothing we've read in the argument suggests otherwise.

Compare this to choice (A):


This choice recognizes that medicinally valuable substances have already been discovered in tropical rain forest plants. What the conclusion really requires is that some substances in tropical rain forest plants are different from the ones already discovered.

Choice (A) calls out this requirement, as obvious as it might seem. And this choice doesn't slip up by saying something like, "All substances of medicinal value contained in tropical rain forest plants not yet studied by scientists differ from those substances already discovered in tropical rain forest plants." That change in wording would make this answer choice just as wrong as (D).

That's why we eliminate (D) and stick with (A).

I hope this helps!