Problem: What are the most important words?
lives were extinguished when those planes crashed into the twin towers .
Answer: crash, plane, tower

Problem: What are the most important words?
A few jumbo jet planes parked in a waiting area of an airport.
Answer: airport, area, park, plane

Problem: What are the most important words?
an ethnically mixed class of pupils at a school
Answer:
class, pupil, school