Which three of these statements about penultimate hop popping are true？（）
- AIt is used only for directly connected subnets or aggregate routes.
- BIt can only be used with LDP.
- CIt is only used when two or more labels are stacked.
- DIt enables the Edge LSR to request a label pop operation from its upstream neighbors.
- EIt is requested through TDP using a special label value that is also called the implicit-null value.
- FIt is requested through LDP using a special label value that is also called the implicit-null value.