How does the $80/20$ rule apply to coding and software development? a$.$ It suggests that $80\%$ of coding time should be spent on testing and debugging. b$.$ It states that $20\%$ of code should be written by senior developers, while $80\%$ can be written by junior developers. c$.$ It emphasizes that $80\%$ of the code should focus on core functionality, while $20\%$ can handle edge cases. d$.$ It implies that $80\%$ of software development can be completed in $20\%$ of the estimated time.