24. Find the lower limit of γ-ray frequency for pair production (electron-positron creation).

Note: electron and positron mass mₑ = 9.1 × 10⁻³¹ kg

          speed of light c = 3 × 10⁸ m/sec

          Planck's constant h = 6.6 × 10⁻³⁴ J-sec
(A) 2.0 × 10²⁰ Hz
(B) 2.5 × 10²⁰ Hz
(C) 3.0 × 10²⁰ Hz
(D) 3.5 × 10²⁰ Hz
(E) 4.0 × 10²⁰ Hz

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