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 LD
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.