I want to understad how to make the calculations to find out the corridor...please

Can you explain me with details

Hicks Cable Company has a defined benefit pension plan. Three alternative possibilities for pension- related data at January 1, 2013, are shown below: Case 1 Net loss (gain-AOCI, Jan. 1 2013 loss (gain) on plan assets 2013 loss (gain) on PBO Accumulated benefit obligation, Jan. 1 Projected benefit obligation, Jan. 1 Fair value of plan assets, Jan. 1 Average remaining service period of active employees (years) $ 320 (11) (23) (2,950) (3,310) 2,800 12 ($ in 000s) Case 2 $ (330) (8) 16 (2,550) (2,670) 2,700 15 Case 3 $ 260 2 (265) (1,450) (1,700) 1,550 10 For Case 1, what is the corridor amount associated with determining the Excess? ($ in 000s) A. $331 B. $320 C. $270 D. $170