In the case of a defined benefit plan, why the actuarial gains may arise?

a. The actual returns on plan assets are less than the expected returns. b. The actual returns on plan assets are greater than the expected returns. c. There are adverse differences between actuarial assumptions made at the end of the previous period and actual events which have occurred in the current period d. There are adverse changes in actuarial assumptions between the start and the end of the current period