20. The lifetime of a muon is 2.2 μs. Suppose that a muon is generated at a height of 15 km above Earth's surface. In order to be detected on the ground, the velocity of the muon must be at least (c = 3 × 10⁸ m/s²)
(A) 0.9 c
(B) 0.99 c
(C) 0.999 c
(D) 0.9999 c.

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