The correct answer is
C.
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If a uranium atom absorbs a neutron it will be unstable, and will generally split into two fragments. This process, the splitting of a large nucleus into two smaller ones, is known as NUCLEAR FISSION.
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Velocity = frequency*wavelength
frequency = velocity/wavelength
= (3*10^8)/(4.257*10^-7)
= 7.047*10^14