$1.$ Between $2002$ and $2007,$ while under the protection of the Bankruptcy Court, Delta Airlines and Northwest Airlines froze its employee pension plans. What does this mean?

a$.$ The pension plan is preserved and accrued benefits will be paid, but the airline will no longer be required to pay into the plan

b$.$ The pension plan and accrued benefits are totally eliminated

c$.$ The pension plan is replaced by a $401$K plan

d$.$ The pension plan is eradicated but the accrued benefits will be paid back to retirees via the aviation trust fund $($ATF$)$