In Option A, the center bore in the hub is produced by a two step machining (rough drill & finish machine) operation. 

With the large diameter of the center bore and the depth of the hub, a comparative cost analysis showed that  two step machining is too expensive and time consuming, compared to using a center core and a finish machining operation. 

In addition, with a solid hub, the center section is heavy enough that it will be more difficult to cast without shrinkage porosity and internal flaws.