book 2 schweser pg 60--pension plan asset betas

I get the jist of this stuff that the WACC can be overstated…but how they got to that conclusion with everything on this page I dont get…First, for a pension plan, why as D/E increases, the risk decreases??? should risk increase as you increase debt? Also, why are the debt betas of pension plan assets assumed to be zerio as stated in the second paragraph on the page?