Which three of these statements about penultimate hop popping are true?

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

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.

Explanation:
Pass4sure sure says DEF

To implement penultimate pop the edge LSR requests a lable pop from upstream neighbor via LDP OR TCP using special implicit-null lable 3 for LDP 1 for TCP; others agree and reference http://www.ciscopress.com/articles/article.asp?p=680824&seqNum=2

Other sources say AEF and refer to http://fengnet.com/book/mpls_vpn/ch02lev1sec3.html

It looks like they got this right from my MPLS bible (MPLS Configuration on Cisco IOS Software Page 17)
(PHP)This process is signaled by the downstream Edge LSR during Label distribution with LDP. The downstream Edge LSR distributes an implicit-null (POP) label to the upstream router, which signals it to pop the top label stack
So answers D and F are correct for sure.
That leaves the question does TDP do this? My reference shows only LDP, but these two distribution protocols are very similar, so thats a tough one, so now ask yourself Is there a better answer??
And the better answer is really A, PHP is essentially used to prevent an additional IP lookup, which comes from having to first look in the LFIB, and then the FIB. When the edge LSR knows the destination is unlabled it can request the top label in the label stack to be popped, so it doesnt have to look in the LFIB and LIB. Now it can only get an implicit-null label assigned to it if the destination network is local(directly connected) or an aggregate route or what we call an unlabeled destination.

So I think A is a better answer. Overall not a well written question. But if it makes anyone feel better I see the exact same question on other study material saying ADF.