The correct answer is D. both a and b.
A learning curve is a graphical representation of the improvement in the performance of an individual or group as a result of practice or experience. Learning curve models are used to predict the amount of time or effort required to complete a task or project as a function of the number of times the task has been performed.
The cumulative average time learning model is a type of learning curve model that assumes that the amount of time required to complete a task decreases by a constant percentage each time the task is performed. The incremental unit time learning model is a type of learning curve model that assumes that the amount of time required to complete a task decreases by a constant amount each time the task is performed.
Both the cumulative average time learning model and the incremental unit time learning model are useful for predicting the amount of time or effort required to complete a task or project. However, the cumulative average time learning model is more accurate for tasks that are performed a large number of times, while the incremental unit time learning model is more accurate for tasks that are performed a small number of times.