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

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

A. Unit
B. Measurement Type
C. Index
D. Measured Characteristic

Answer: Unit