When a beam of accelerated electrons hits a target, a continuous X-ray spectrum is emitted from the target. Which of the following wavelengths is absent in X-ray spectrum, if the X-ray tube is operating at 40,000 volts?

• A

  $0.25 \stackrel{\mathrm{o}}{\mathrm{A}}$

• B

  $0.5 \stackrel{\mathrm{o}}{\mathrm{A}}$

• C

  $1.5 \stackrel{\mathrm{o}}{\mathrm{A}}$

• D

  $1.0 \stackrel{\mathrm{o}}{\mathrm{A}}$