Air is cooled and
becomes denser.
Denser air sinks.
The process repeats, producing a (i wasn't sure about this one)
Air is heated and becomes less dense.
Less dense air rises.
hope this helps!
I say 1 it seems the most accurate
Answer:

Explanation:
given,
wavelength, λ = 50 cm
speed, v = 4 m/s
Amplitude, A = 5 cm
general equation of the wave along x- axis

sign is positive when wave is traveling in negative direction
now,



inserting all the values

Hence, the y-equation of wave is equal to 
1 watt = 1 joule/sec
1800 watts = 1800 joules/sec
(1800 J/sec) x (20 min) x (60 sec/min)
= (1800 x 20 x 60) joules = 2,160,000 joules .
( 2.16 megajoules )