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The threshold level for logic 1 in the TTL family is

1. any voltage above 2.5 V

2. any voltage between 0.8 V and 5.0 V

3. any voltage below 5.0 V

4. any voltage below V$_{cc}$ but above 2.8 V

(D) any voltage below Vcc ( ≈ 5 V) but above 2.8 V
Transistor transistor logic (TTL) was designed that any input from 0.8 volts to 0 volts is considered a logic 0, and any input from 2.6 to 5.0 volts is considered a logic 1. See the following Wikipedia page for more information
http://en.wikipedia.org/wiki/Transistor_...

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