Which of the following statements best describes the operation of a synchronous up-/down-counter?

• The counter can count in either direction, but must continue in that direction once started.
• The counter can be reversed, but must be reset before counting in the other direction.
• In general, the counter can be reversed at any point in its counting sequence.
• The count sequence cannot be reversed, once it has begun, without first resetting the counter to zero.