What are the keywords in the following sentence? the clock and roof at tube station
clock, roof, station

What are the keywords in the following sentence? flock of sheep in a field
field, flock, sheep

What are the keywords in the following sentence? christmas trees discarded after the festive season await collection on the street
collection, discard, street, tree

What are the keywords in the following sentence? crows flying away from a tree in the evening
crow, fly, tree