Task: Property Constraint Reasoning (ontology: Earth_Environment/SWEET)

The property 'was performed by' applies to instances of 'process', and 'chemical process' is a subclass of 'process'. Which class can appear as the value of 'was performed by' for instances of 'chemical process'?

A. tropopause
B. heavy snowfall
C. book end vorticities
D. agent

Answer: agent