Of the following statements concerning network switching systems, which statement applies to the frame relay method?
a) 53-byte fixed-length frames are used.
b) Variable-length frames are used and transmission error control within the network is simplified.
c) A store-and-forward system performs transmission error control including retrasmission between exchanges within the network.
d) When terminals are connected using the frame relay protocol, they can communicate using protocols other than the frame relay protocol.