He went … the corridor, looking for the principal's office.

Analyze the question: We need a preposition that fits "went ___ the corridor, looking for the principal's office." The person is moving through a long, narrow passage, searching for a room.

Explain the concept: Prepositions of movement show the path a person takes.

• along means moving in a line beside or through a long narrow place, such as a road, river, corridor or path.
• under means beneath something, which doesn't suit a corridor.
• among is used with groups of three or more people or things (e.g., among the students), not with a single passage.
• around means circling or surrounding something, or moving in various directions. It could work in "around the building," but "went around the corridor" sounds odd and doesn't carry the idea of walking down its length.

Solution:

1. A corridor is a long, narrow passage.
2. The person is walking down it to find an office, so the movement follows its length.
3. "Along" best expresses that movement.

"He went along the corridor, looking for the principal's office."

Check against the options:

• A. under: wrong, since it means beneath.
• B. among: wrong, since it needs a group.
• C. along: correct.
• D. around: not natural here.

Exam tip: When you see words like corridor, road, street, river, path, think along. It is the standard preposition for moving down the length of something.