What is the difference between deep packet inspection and stateful inspection?

What is the difference between deep packet inspection and stateful inspection?
A . Deep packet inspection is more secure than stateful inspection on Layer 4
B. Stateful inspection verifies contents at Layer 4 and deep packet inspection verifies connection at Layer 7
C. Stateful inspection is more secure than deep packet inspection on Layer 7
D. Deep packet inspection allows visibility on Layer 7 and stateful inspection allows visibility on Layer 4

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Answer: D