Why would the data bus and address bus be incomplete without the control bus?

Correct answer: Because the control bus provides read, write and timing signals

The address bus identifies where a transfer should occur, and the data bus carries the actual information. The control bus supplies signals such as read, write, interrupt, and timing signals that tell devices what operation to perform and when. Without these coordinating signals, an address and data value would not form a properly controlled transaction. It is not primarily a storage, display, or power bus.