What are the keywords in the following sentence? this tiny wooden plaque nailed to a tree is all that alerts hikers that they have crossed .
cross, hiker, tree

What are the keywords in the following sentence? a healthy coral reef surrounded by reef fish
fish, reef, surround

What are the keywords in the following sentence? skyline seen at sunset as thick clouds cover the city
city, cloud, cover, sunset

What are the keywords in the following sentence? portrait of a sheep lying on the grass

grass, lie, portrait