wacc and hedging pension liablity

in wacc formula for pension plan, I found it is equal to cost of equity, so in the exam, I use cost of equity as a proxy for wacc, should be fine?

regarding hedge pension liability, why we should use derivative to hedge? if I simply buy the long term bond, does it a good hedge against liability? also the curricum mentions using derivative plus asset only approach to hedge(last question in the last curricum reading of Volume 2 textbook, but can not using diversified portfolio to hedge pension liability, I want to know why diversified portfolio can not be used in hedging pension liability risk?

why diversified portfolio can not be used in hedging pension liability risk?