Gamma rays, or rather gamma radiation are most commonly used.
Answer:B
Explanation:Coefficient represents the number of units of each substance.
Explanation:
a positively charged nucleus is surrounded by mostly empty space.
<u>Answer:</u> The wavelength of spectral line is 656 nm
<u>Explanation:</u>
To calculate the wavelength of light, we use Rydberg's Equation:

Where,
= Wavelength of radiation
= Rydberg's Constant = 
= Final energy level = 2
= Initial energy level = 3
Putting the values in above equation, we get:

Converting this into nanometers, we use the conversion factor:

So, 
Hence, the wavelength of spectral line is 656 nm