REMOVE

1. plants
2. workers
3. men (male humans)
4. railroads

WHAT

1. Visual Works (hierarchy n … (1)
2. arbors (1)
3. artists (visual artists) (1)
4. atmospheric perspective (1)
5. automobiles (1)

More 

WHERE

1. Westminster, Maryland (1)

WHO

1. Campbell, Sandra L. Still (1)
2. Sandra L. Still Campbell; … (1)

WHEN

1. 1961 (1)
2. 1965 (1)
3. between 1961 - 1965 (1)