问题：

[单选] 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 block