A  is correct because RSTP alternate ports block traffic while receiving superior BPDUs from a neighboring switch.  An alternate port is a backup port for a root port, which means it receives better BPDUs from another bridge than the current root port 1 .  However, an alternate port does not forward any traffic, as it is in a discarding state 2 . It only listens to BPDUs and waits for the root port to fail.  If the root port fails, the alternate port can immediately transition to a forwarding state and become the new root port 1 .

C  is correct because RSTP alternate ports provide an alternate higher cost path to the root bridge.  An alternate port is selected based on the same criteria as the root port, which are the lowest bridge ID, the lowest path cost, the lowest sender port ID, and the lowest receiver port ID 3 .  However, an alternate port receives a higher cost BPDU than the root port, otherwise it would be the root port itself 1 . Therefore, an alternate port provides an alternate higher cost path to the root bridge than the root port.