Grace is a first$-$time project manager analysing a project schedule. She intends to apply resource levelling and has been given the following guidelines. Which of these four statements is incorrect?

a$.$

Resource leveling is applied to a schedule before it has been analysed by the critical path method.

b$.$

Resource levelling is applied to keep resource usage at a constant level.

c$.$

Resource levelling can often cause the critical path to change.

d$.$

Resource leveling is necessary when resources have been over$-$allocated.