What are the keywords in the following sentence? the only church standing in a city
church, city, stand

What are the keywords in the following sentence? the relative size of the planets in star system
planet, size, system

What are the keywords in the following sentence? an orange building a clock and a blue sky
building, clock, sky

What are the keywords in the following sentence? the plagued train exiting central station on its way
exit, station, train