May/June 2004. Question 27.
I guess it's just estimation.
speed of light : 3 x 10^ 8 m/s
wavelength of infrared : 1000 nm to 100000 nm approx. (let's assume, you've studied approximate wavelengths of the different radiations)
use v= f x wavelength
3 x 10^14 and 3 x 10^12.....the values u get for frequency for the wavelengths mentioned above. So, I guess it's ,safely, C.
However , I do hope someone can suggest a much simpler way, sans calculation, hopefully...