The most common metal in the Earth’s crust, aluminum (or aluminium) wa

To answer this question, we must infer from facts given about iron. We are told that “many other metals, such as iron, are less reactive than aluminum, but their superficial oxides do not form as swiftly, completely, or impermeably.” Since we are also told that the oxide layer on aluminum is “thin but robust” and “prevents further corrosion,” we can logically infer that the oxide layer on iron does not prevent further corrosion as effectively as the layer on aluminum, and therefore iron in all likelihood corrodes more quickly than aluminum.

(A) CORRECT. This answer choice states our inference clearly.
(B) We can indeed conclude that iron oxides form more slowly (“not as swiftly”), but we cannot conclude that iron oxides form more robustly than aluminum oxides. In fact, we are told that aluminum oxide forms a “thin but robust… seal,” while iron oxides, among others, “do not form as… completely, or impermeably.” If the iron oxides are less complete and less impermeable, then we cannot infer that they are more robust; if anything, we should assume that they are less robust.
(C) We know that iron is less reactive than aluminum, but we cannot conclude that iron is therefore cheaper to isolate from its ores. The extreme reactivity of aluminum was the reason for the difficulty and expense of separating aluminum from its ores by traditional chemical means, but iron could also be expensive to extract from its ores by traditional chemical means; we simply don’t know enough about the extraction process to make an inference here.
(D) This choice is directly contradicted by the passage: it is aluminum that forms a more effective “passivization” seal, not iron.
(E) Although iron is less reactive than aluminum, we do not know whether it is more likely to be found in its isolated, elemental state. If anything, because it does not form as effective passivization seals, we would expect pure iron to corrode away in nature.

This question asks us about a specific detail mentioned in the passage: the “passivization” layer on the surface of aluminum metal. The passage indicates that this layer: (1) is formed from the combination of atmospheric oxygen with the pure metal, (2) is thin but robust, and (3) prevents further corrosion. We should look for an answer that matches one of these facts.

(A) This choice reverses the stated cause-and-effect sequence. The layer is the result of the reaction between oxygen and aluminum—not the cause of that reaction.

(B) We are told that this layer is “thin but robust”; in fact, in the next sentence, we are told that the superficial (= surface) oxides of iron “do not form as swiftly, completely, or impermeably” (as the surface oxides of aluminum). Thus, the passivization layer on aluminum is actually more impenetrable than the one that forms on iron.

(C) We cannot conclude that this layer “lowers the utility” or usefulness of pure aluminum. If anything, we would guess that this layer, because it prevents further corrosion, makes pure aluminum very long-lasting and therefore more useful—and that guess would be an inference anyway.

(D) CORRECT. The passivization seal of aluminum oxide “prevents further corrosion” in aluminum, while “many other metals” do not form such seals “as swiftly, completely, or impermeably.” The passivization layer, then, provides aluminum with at least one advantage relative to other metals.

(E) We do not know whether this layer is what causes the “traditional” purification of aluminum to be an expensive process.

To draw an economic comparison or analogy between what happened in 1886 (according to the passage) and a hypothetical situation, we first need to understand the significance of the stated events in that year. The passage tells us that, in 1886, two inventors developed a new process to isolate aluminum, and that as a result the price plummeted. We might predict that we are looking for a situation in which a previously expensive product suddenly becomes cheaper because of a new technology or process.

(A) While this situation captures a couple of the features of the events of 1886 (two researchers working independently make a discovery at the same time), these features do not imply anything about the economic impact of that discovery.

(B) This situation is in some ways opposite to the events of 1886. In this choice, a product containing one material (lead) is replaced by a product containing a “much rarer” material (lithium); if anything, we would expect the price of the product to go up, not down.

(C) In this choice, nothing is indicated or implied about the economic impact of replacing an old process (electronic signal processing) with a new process (direct processing of light signals). We do not know whether the new process would be cheaper or more expensive.

(D) This situation is in some ways opposite to the events of 1886. Here, a commodity becomes scarce, and the price shoots up.

(E) CORRECT. Expensive diamonds become less expensive due to the perfection of a new technological process: “low-cost artificial synthesis.” This situation would be directly analogous, in terms of economic impact, to what happened with aluminum in 1886.