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