Hot zone. Air lock. These are both examples of jargon, aka slang.
A functional text is a text to fulfill a certain function, such as instruction manual or description of getting somewhere. A dialogue would be too confusing and redundant for this purpose; the best functional text would simply describe the information it needs to provide in a clear manner.
The right answer here would be A - the phrase 'modulate your voice'
refers to a need to vary your voice's properties. Modulating means
changing the pitch or tone, and can be used referring to one's voice, or
to an instrument. You can modulate both up or down, and this usually
involves changing the key signature in which you are playing or singing.
The correct answer is 'their'
