Consider a VC network with a 2

Consider a VC network with a 2-bit field for the VC number. Suppose that the network wants to set up a virtual circuit over four links: link A, link B, link C, and link D. Suppose that each of these links is currently carrying two other virtual circuits, and the VC numbers of these other VCs are as follows:

In answering the following questions, keep in mind that each of the existing VCs may only be traversing one of the four links.
a. If each VC is required to use the same VC number on all links along its path, what VC number could be assigned to the new VC?
b. If each VC is permitted to have different VC numbers in the different links along its path(so that forwarding tables must perform VC number translation), how many different combinations of four VC numbers(one for each of the four links )could be used?