There should be a funciton, which can ...
When the input score is >= 90 and < 100, return "S"
When the input score is >= 80 and < 90, return "A"
When the input score is >= 70 and < 80, return "B"
When the input score is >= 60 and < 70, return "C"
When the input score is >= 0 and < 60, return "D"
Jasmine 2.4 API refenence (http://jasmine.github.io/2.4/introduction.html)