[单项选择]When is a first-hop redundancy protocol needed in the distribution layer?() A. when the design implements Layer 2 between the access arid distribution blocks B. when multiple vendor devices need to be supported C. when preempt tuning of the default gateway is needed D. when a robust method of backing up the default gateway is needed E. when the design implements Layer 2 between the access switch and the distribution blocks F. when the design implements Layer 2 between the access and distribution blocks