Why are DNS open resolvers the focus of so many malicious activities?

Why are DNS open resolvers the focus of so many malicious activities?
A . DNS open resolvers sit inside the enterprise firewall which must pass DNS traffic making them easy attack vectors.
B . These servers must respond to any query from any Internet host.
C . Since DNS open resolvers belong to no administrative authority, they cannot be secured.
D . DNS open resolvers are used by enterprises allowing the attacker an easy means to infiltrate the enterprise network.

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Answer: B