Self-study
When a stone is trimmed by a mason and exposed to the elements, a coating of clay and other minerals, called rock varnish, gradually accumulates on the freshly trimmed surface. ███████ ██████ ███████ ███████ ███ ███████ ██ ██████ ██ ██ ██████ ████████ ███ █████ ██ ██ ████ █████ █████ ████ █████ ███ ███████ ██████ ████ ████ █████ ██ ███ █████ ███████ █████ ██ ███ ████████ ██ ███████ ████ ███ ████████ ███ █████ ████ ██████ ███ ███████ ██ █████████ ██ ███ ████████ ██ █████
Argument Structure
After some context explaining what "rock varnish" is, the stimulus tells us that organic matter trapped underneath the rock varnish on the stones from an Andean monument is over 1,000 years old. Because this organic matter grew on the stones shortly after they were trimmed, the stimulus concludes that the monument predates the European arrival in the Americas in 1492.
Assumptions and Strategy
For this Weakening question, you might speculate that a good alternative explanation would be that the organic matter arrived or was trapped under the varnish for other reasons than "growing on the stone", and so could be older than whenever the stones were trimmed. But notice that the idea that the organic matter grew on the stone "shortly after it was trimmed" is one of the premises, so we'll have to take that as a given.
The key assumption here is a little more subtle — I actually didn't catch it on my first go-around until I looked over the answer choices. Especially in testing conditions, if you're having trouble pre-phrasing, it can be best to go to the answer choices and let them narrow down the range of possibilities.
18.
Which one of the following, ██ █████ ████ █████████ ███████ ███ █████████
a
Rock varnish itself ████████ ████ ███████ ███████
This might be tempting. You might be thinking that if rock varnish itself contains some organic matter, then maybe the organic matter, while from some older source, accumulated on the monument after 1492. But this thinking doesn't work for two reasons: first, we're talking about the organic matter found underneath the rock varnish layer, not in the rock varnish layer. Second, we're taking it as a given (one of the premises) that the organic matter found underneath the rock varnish grew on the stones shortly after they were trimmed. So even if the rock varnish contains some organic matter, that doesn't do anything to explain the presence of the organic matter underneath the varnish, which grew on the stones after trimming — and so this answer choice doesn't weaken the argument.
b
The reuse of ███████ ███████ ██████ ███ ██████ ██ ███ █████ ████ ██████ ███ █████ █████
Correct. This answer choice takes advantage of an important assumption in the argument. Notice that the argument about the organic matter makes a very strong case that the stones were trimmed over 1,000 years ago — but the stimulus then jumps to a conclusion about when the monument was built. In other words, the argument assumes that the monument was also built shortly after the stones were trimmed. But if this answer choice is true, then that assumption is undermined. Even if the stones themselves are ancient, that doesn't prove anything about the age of the monument itself, which could have been built after 1492.
Alternate explanation
Alternate explanation
Weaken: Introduce or support an alternate explanation for a phenomenon.
Strengthen: Helps to eliminate an alternate explanation for a phenomenon.
c
The Andean monument █████ █ ████████ ███████████ ██ █████████ █████ ██ ███████ █████ ██ ███████ █████
Irrelevant. Knowing that the monument is similar to other "ancient" sites in other parts of the world doesn't do anything to this argument.
d
The earliest written █████████ ██ ███ ██████ ████████ █████ ████ █████
Irrelevant. Just because the earliest written reference is from after 1492 doesn't mean the monument is also from after 1492. There could have been earlier written records that have been lost, or perhaps the culture that built the monument didn't have writing.
e
Rock varnish forms ████ ███████ ██ ██ ████ ██ ███████ ██████ ████ ███ ██████ ██ █ ████ █████████ ██████
Irrelevant. We don't know that this monument was found in a "dry, sheltered" place, so we don't know if this fact even applies to the monument. Even if it did, we know that rock varnish did develop on this monument, and that it trapped organic matter underneath it that was over 1,000 years old. This answer choice doesn't provide an alternative explanation of that fact to what the stimulus provides, so it doesn't weaken the argument.