Which statement differentiates a time constraint from a duration constraint?

Which statement differentiates a time constraint from a duration constraint?

• A. Time constraint applies to a single event; duration constraint applies to a pair of events
• B. Time constraint applies to a pair of events; duration constraint applies to a single event
• C. Time constraint is optional; duration constraint is mandatory
• D. Time constraint relates to guards; duration constraint relates to state invariants

Answer: (A)

The main idea here is how timing constraints are scoped. A time constraint specifies when a single event must occur—its moment in time. A duration constraint specifies how long should pass between two events, meaning it ties together a start and an end event and controls the elapsed time between them. That’s why the correct statement is that a time constraint applies to a single event, while a duration constraint applies to a pair of events. For example, you might require a task to start by a certain deadline (one event’s timing). In contrast, you might require that once a request is made and a response is delivered, the time between those two events stays within a limit (two events involved). The other ideas don’t capture the essential difference: time constraints do not inherently involve two events, and duration constraints are all about the relationship between two events. The notions of optional versus mandatory or any relation to guards or state invariants aren’t what distinguishes time versus duration constraints.

The main idea here is how timing constraints are scoped. A time constraint specifies when a single event must occur—its moment in time. A duration constraint specifies how long should pass between two events, meaning it ties together a start and an end event and controls the elapsed time between them.

That’s why the correct statement is that a time constraint applies to a single event, while a duration constraint applies to a pair of events. For example, you might require a task to start by a certain deadline (one event’s timing). In contrast, you might require that once a request is made and a response is delivered, the time between those two events stays within a limit (two events involved).

The other ideas don’t capture the essential difference: time constraints do not inherently involve two events, and duration constraints are all about the relationship between two events. The notions of optional versus mandatory or any relation to guards or state invariants aren’t what distinguishes time versus duration constraints.