Hi!
The answer is 3. All these answers are correct.
Practical radio communication was furthered by enthusiasts who were experimenting with Hertz and Maxwell's findings.
By the theory of modulation because it helped establish the earliest method to transmit voice by radio.
By the use of vacuum tubes because they made detection and amplification of radio signals possible.
So, all of these 3 factors furthered the development of practical radio communication.
