What is with the grade system to get an s

Ok i was playing yasuo mid and we lost unforcenetly but i thout i was going to get and s instead i got an A while my K/D ratio was 15/9/5 218 farm 35,000 dmg dealt to champions (max in game) triple kill largest killing spree 5 and i still got and A instead of an S what the heck man how much more am i suppost to do? Like what more is there i got 3-4 towers placed a lot of wards and no s i would like to get my lvl 6 token riot cause that makes no sence if it does then explain to me please i dont understand the S rank system
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