Problem: What are the most important words?
police officers stand guard at the site of an explosion
Answer: guard, officer, site, stand

Problem: What are the most important words?
the largest container ship to ever make port unloads its cargo .
Answer: cargo, port, unload

Problem: What are the most important words?
a grizzly bear before it goes to its den to sleep until spring .
Answer:
bear, den, sleep