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

The property 'has source unit' applies to instances of 'Unit Conversion', and 'GradToRadian' is a subclass of 'Unit Conversion'. Which class can appear as the value of 'has source unit' for instances of 'GradToRadian'?

A. Characteristic
B. CategoricalStandard
C. Measurement Type
D. Unit

Answer: Unit