Answer Explanation

[PASSAGE] "Karst is a distinctive and dramatic type of landscape that forms from the dissolution of soluble rocks, including limestone, dolomite, and gypsum." and "The carbonic acid slowly dissolves the calcium carbonate, transforming it into calcium bicarbonate (Ca(HCO₃)₂), which is soluble in water and is carried away by the flow of groundwater." These sentences state that soluble rocks are dissolved, and that a soluble substance (calcium bicarbonate) is dissolved in water. [WHY CORRECT] Option 1, "capable of being dissolved," is correct because the passage explicitly links "soluble" with the process of "dissolution" and "dissolves." The text describes "soluble rocks" forming through "dissolution" and later states that calcium bicarbonate "is soluble in water and is carried away by the flow of groundwater," directly indicating its ability to be dissolved. [TRAPS] Option 2, "difficult to break," is incorrect because being soluble refers to chemical transformation, not physical breakage. The rocks are dissolved, not broken. Option 3, "containing valuable minerals," is incorrect; while some rocks might contain valuable minerals, the term "soluble" specifically describes how a substance reacts with a liquid, not its intrinsic value. [TIP] When encountering a vocabulary question, first try to understand the word's meaning from its context in the passage. Look for clues like synonyms, antonyms, or explanations within the surrounding sentences. If context is insufficient, consider the word's root or common usage.

Why the Other Options Are Incorrect

• Option B (“difficult to break”) is too broad to capture the contextual meaning.
• Option C (“containing valuable minerals”) shares a root yet shifts the tone of the sentence.
• Option D (“visible from a great distance”) fits a different collocation than the one used here.

Key Language in Context

Watch how academic vocabulary around Geology signals meaning. TOEFL vocabulary items reward readers who track tone, transitions, and precise word choice rather than surface keywords.

Skill Takeaway

Let the surrounding clause, not the word in isolation, decide the meaning.