What is responsible for the production of x ray emission at the cutoff wavelength?

1)the last emission in a series of many emissions by the incident electron as it loses its energy.

2)the first emission in a series of many emissions by the incident electron as it loses its energy.

3)the most energetic emission in a series of many emissions by the incident electron as it loses its energy.

4)the least energetic emission in a series of many emissions by the incident electron as it loses its energy.

5)the emission by the incident electron when it loses its energy in a single emission