b	B,  Headers only contain information about local destinations. Ethernet was originally designed for a local network so the headers only contain information about sites on the local network. For this reason the data would never travel distances beyond the local network. Twisted cable which is prone to error and loss can be used over long distances with the inclusion of repeaters that are designed to correct errors. 
c	C, All the others are fundamental to ATM. Though optic fibre is a preferred medium of transport, it is not essential to the process. In fact, satellites can be used instead.
d	D,  There are 40 Data bytes in an AAL5 cell.  The total cell size is 53 bytes = 8 bytes CRC trailer + 1 byte HEC + 4 bytes descriptor + 40 data bytes. In nonAAL5 there are 48 data bytes, since there is no 8 bytes for the CRC trailer.
a	A,  The Midway enqueues completed PDUs into the In_Service list. The PDUs being reassembled are in the Reassembly_queue.  The VCI descriptor points to the next buffer/queue slot to enqueue a receive_descriptor on.  The PDUs are not transmitted by the reassembling engine, but are received by the reassembling engine.
b	B,The Midway interrupts the Host Interface Block to signal that a PDU is completed and ready to be DMAed, the Host Interface Block will dequeue the VCI_number from the In_Service queue and enqueue a DMA_Descriptor in the DMA_Received_queue.  The DMA Interface block will dequeue the DMA_descriptors, get the Readptr from the VCI_Descriptor pointed to and DMA the PDU.  The PDUs are not transferred  directly to the host RAM by the Host Interface Block, but via the DMA Interface Block.  The DMA interface transfer the PDU, the point of DMAing is to avoid interrupting the host for data transfers.
c	C,  The controller interrupts the host after every transfer.  The message is DMAed, so the host does not need to be interrupted to transfer data. It is true that an over flow or an underflow will generate an error and cause an interrupt, but this is not the only time.  There is no HEC bit in Ethernet, and it is a byte in ATM.
d	D,  Only the ethernet can retransmit lost packets, the ATM does not retransmit for lost packets, collisions, etc.  ATM sends a packet and hopes that it gets there.