If not connected to a ground or logic high signal TTL inputs A) are invalid. B) are logic low. C) are considered logic high. D) automatically go to Hi-Z mode.?

If not connected to a ground or logic high signal, TTL inputs A) are invalid. This means that the input may pick up noise and could fluctuate between states, leading to unreliable circuit behavior. It's important to properly terminate unused inputs to prevent them from floating and causing undefined states.