BOM Question.

Ok So we buy an extruded length called Part A

From Part A we make Part B and in the BOM it calls up Part A. (modelled as part in part)

Sometimes we make part C from part B. (modelled as part in part)

How do I get The BOM on Part C to call up Part B and not Part A.

Thanks in advance

SolidworksDrawings And Detailing