填空题

There are three different types of software maintenance.Firstly,there is maintenance to repair software _____.Coding errors are usually relatively _____ to correct; design errors are more ______ as they may involve rewriting several program components.Requirements errors are the _______ expensive to repair because of the extensive system redesign that may be ______.________,there is maintenance to adapt the software to a _____ operating environment. This type of maintenance is required when some _____ of the system's environment such as the hardware, the platform operating _____ or other support software changes. The application system must be _____ to adapt it to cope with these environmental changes. And thirdly,there is ______ to add to or modify the system's functionality. This ____ of maintenance is necessary when the system requirements change in ______ to organizational or business change. The scale of the changes required to the ________is often much greater than for the other types of maintenance. In _____ , there isn't a clear-cut _________ between these types of maintenance. When you _____ the system to a new environment, you may add functionality to take advantage of new _______ features.Software faults are often exposed because users use the system in unanticipated _____. Changing the system to ______ their way of working is the best way to fix these faults.