How to get a D rank??

I just had the internet from hell - my ping went up to 500 and stayed there, I had to quit to trouble shoot after about 8 minutes. After 20 minutes of of scratching my head and looking confused at my modem I reconnected. Then I get back into the game to find all 3 inhibs down and no chance of rescuing it - :( But I also got camped hard in those first few minutes died several times, got about 60cs over a 30 minute "game", died 2-3 times of the end because Ez ult took 75% of my hp and stuff. End of game I was about 1/6/2 with 60 cs as an ADC and I still got a C, not even a C-. How in the name of all that is holy do you get a D??
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