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

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

A. CategoricalStandard
B. Identifying Characteristic
C. Composite Characteristic Qualifier
D. Unit

Answer: Unit