_Base AD is the portion of attack damage originating from the champion itself, consisting of a constant base value that is individual for every champion and a component granting AD on leveling up. _
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_Bonus AD is the portion of attack damage originating from runes, masteries, abilities, aura effects and, most importantly, items. _
The answer is A
In other words, AD that a champion starts with and gains as they level is base AD.
Bonus AD is from external sources not included from leveling up.
Source: http://leagueoflegends.wikia.com/wiki/Attack_damage
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It is not 50% of her critical rate. Her crit rate is the bonus damage.
This is the formula.
50% + (critical chance × (1 + 0.5 × bonus critical damage))% AD bonus physical damage, modified to 150% AD versus minions and monsters.
If Caitlyn has 80% crit chance, it would be 80 additional damage.
I plugged in the formula so it would be 170 additional damage, but I think it is 80.
Source: http://leagueoflegends.wikia.com/wiki/Caitlyn