18. In an x-ray diffraction measurement, if first-order reflection occurs in a crystal at Bragg angle θ₁, at what Bragg angle θ₂ does second-order reflection occur from the same family of reflecting planes? θ₂= (A) 2θ₁ (B) 2sinθ₁ (C) sin⁻¹(2θ₁) (D) sin⁻¹(2sinθ₁) (E) None of the above