21. A photon whose wavelength is 2.48×10⁻¹¹ m is scattered off an electron at an angle of 90° in a Compton experiment. What is the wavelength of the scattered wave? (mₑ = 9.11×10⁻³¹ kg; h = 6.63×10⁻³⁴ J·s; c = 3.00×10⁸ m/s; e = 1.60×10⁻¹⁹ C) (A) 1.24×10⁻¹¹ m (B) 2.25×10⁻¹¹ m (C) 2.72×10⁻¹¹ m (D) 2.40×10⁻¹¹ m (E) 2.48×10⁻¹¹ m