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

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

A. Unit
B. CategoricalStandard
C. Index
D. Identifying Characteristic

Answer: Unit