问题：

[单选] When is a first-hop redundancy protocol needed in the distribution layer?（）

A . when HSRP is not supported by the design
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 switch
F . when the design implements Layer 3 between the access switch and the distribution switch