Power Systems
The inductance of single phase two wire line is given by

 4×10-7× ln(r'/d) H/m
 0.4×10-7× ln(d/r') H/m
 4×10-7× ln(d/r') H/m
 0.4×10-7× ln(r'/d) H/m

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Power Systems
Critical mass is

 none of these.
  minimum mass of fissionable material required to sustain chain reaction.
 maximum mass of fissionable material required to sustain a chain reaction.
 a mass of fissionable material required to stop a chain reaction.

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