Why would you explicitly define the VPI/VCI when deploying cell-mode MPLS?

A.
To reduce the number of labels required.

B.
Because cell-mode MPLS only runs over defined VPI/VCIs.

C.
To ease a migration by connecting an MPLS-enabled ATM network to the old ATM network.

D.
Because MPLS is needed and parts of the private network cross a non-MPLS aware network.

Explanation:

A is the only possible answer. A technique (VC merge) actually exists to help reduce the number of labels (and thus VC) created. by mapping multiple incoming label values to a specific outgoing label, the number of VCs can be reduced. of course i am not sure the meaning of the word "explicitly" since VC merge seem to do all the mapping automatically.
Incorrect Answers:
B: B is false, as the reference states …with some hardware platforms supporting only approximately 1000 VCs. The very small number of VCs supported over an ATM interface makes these circuits a scarce resource that must be tightly controlled. if 1000 is only a small number, and if optionB is true, then according to the question, all 1000 VCs must be explicitly defined. which is not possible, so we may "assume" that VCs can be dynamically assigned.
C: C doesn’t seem to be correct. Explictily defining VCs require careful designs.
D: Option D is wrong, since a LSP requires all
devices to be able to distribute labels, if non-MPLS aware switches are used, they cannot fulfill the downstream-on-demand feature. in this situation, only Frame (but not Cell) mode can be used.