Personal collections
Some sources of electromagnetic waves in free space are listed.
a radio wave transmitter
a source of X-rays
a 
wavelength radar transmitter
a light-emitting diode that emits red light
Which list gives the sources in order of increasing wavelength, from left to right, of the waves emitted by the sources?
1 
3 4 2
2 4 1 3
2 4 3 1
3 1 4 2
