REMOVE

1. workers
2. trains (vehicle groupings)
3. sky
4. trees

WHAT

1. Visual Works (hierarchy n … (1)
2. automobiles (1)
3. buildings (structures) (1)
4. bushes (1)
5. campuses (1)

More 

WHERE

1. San Francisco, California (1)

WHO

1. Clyfford Still; Still, Cl … (1)
2. Still, Clyfford (1)

WHEN

1. 1971 (1)
2. ca. 1971 (1)