Which of these statements about penultimate hop popping are true？（）

• A、It is used only for directly connected subnets or aggregate routes
• B、It can only be used with LDP.
• C、It is only used when two or more labels are stacked.
• D、It enables the Edge LSR to request a label pop operation from Its upstream neighbors
• E、It is requested through TDP using a special label value that is also called the implicit-null value.
• F、It is requested through LDP using a special label value that is also called the implicit-null value.