What are two primary concerns regarding layer two loops in a redundant topology? (Choose two.)

What are two primary concerns regarding layer two loops in a redundant topology? (Choose two.)
A . routing loops
B . costs associated with upgrading from copper to fiber
C . multiple frame copies leading to instability of the MAC address table
D . security issues with a redundant loop indicating to hackers that a back-door exists
E . broadcast storms

Answer: C,D