Below are examples of nonfunctional requirements. Specify which of these

requirements are verifiable and which are not:

$$The system must be usable.$$

$$The system must provide visual feedback to the user within one second of issuing a

command.$$

$$The availability of the system must be above $95$ percent.$$

$$The user interface of the new system should be similar enough to the old system

that users familiar with the old system can be easily trained to use the new system.$$