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

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

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

Answer: Unit