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Difficulty: MediumVerb Tense, Aspect, and Mood

What form of the verb 'expand' correctly completes the sentence to conform to the conventions of Standard English?

Answer:In a 2000 study, researchers placed the single-celled organism *Physarum polycephalum* in a maze containing two food sources. Before they introduced the food, the organism 【had expanded】 throughout the entire maze in search of nutrients. However, once the food was positioned, the slime mold retracted its branches from the dead ends to form a single, efficient path connecting the two sources.

Answer

had expanded
The correct answer is the past perfect form 'had expanded' because the context indicates that the organism's expansion occurred prior to the introduction of the food, which is already described in the past tense ('introduced').

Step-by-Step Solution

1
Identify the timeline established by the other verbs in the passage.
The narrative is set in the past, as shown by the verbs 'placed', 'introduced', 'was positioned', and 'retracted'.
Understanding the primary time frame is essential for choosing the correct tense and aspect.
2
Determine the temporal relationship between the action in the blank and the other actions.
The clause 'Before they introduced the food' establishes that the action of expanding happened prior to another past action ('introduced').
An action that is completed before another point in the past requires the past perfect tense.
3
Conjugate the verb 'expand' into the past perfect tense.
The past perfect form is 'had expanded'.
Past perfect is formed by combining the auxiliary verb 'had' with the past participle of the main verb ('expanded').

Key Concept

Verb Tense and Aspect (Past Perfect)
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Topics

Verb Tense, Aspect, and Mood

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