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

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

A. Observation
B. Characteristic
C. CategoricalStandard
D. Unit

Answer: Unit