challenge: How did the operating system handle I/O devices in the original S/360 and S/370 architectures?
  
  A. Each processor had its own set of I/O channels and addressed I/O devices with a 12-bit cuu address
  B. All processors shared a single set of I/O channels and addressed I/O devices with an 8-bit unit number
  C. Each device had a unique 16-bit address that was used by all processors
  D. I/O devices were addressed using a 4-bit channel number only
  Answer: 