Problem: What are the most important words?
stack of books with the top one open for studying
Answer: book, stack, study

Problem: What are the most important words?
flock of seagulls sitting on a pier at the sea
Answer: flock, pier, sea, seagull, sit

Problem: What are the most important words?
Passenger train going through a section with many tracks.
Answer:
section, track, train