31. With sufficient energy, it’s possible to eject an electron from an inner atomic orbital. A higher-energy electron will then drop into the unoccupied state, emitting a photon with energy equal to the difference between the two levels. For inner-shell electrons, photon energies are in the keV range, putting them in the X-ray region of the spectrum. These characteristic X-rays are labeled with the letter indicating the shell to which the electron drops, followed by a Greek letter indicating the higher level from which it drops; thus ?? designates a transition from the L shell to the K shell. Molybdenum’s X-ray spectrum has its ?? peak at17.4 keV. Note that Plank’s constant is 6.626 × 10⁻³⁴ m² ∙ kg/s, speed of light is 3 × 10⁸ m/s, and one electron charge is 1.6 × 10⁻¹⁹ C. The corresponding X-ray wavelength is closest to
(A) 10 pm
(B) 30 pm
(C) 70 pm
(D) 150 pm
(E) 200 pm.