If blood cannot absorb more nicotine than that present in the cigarettes with lowest level, then no matter which cigarette person smokes, the level of nicotine in all persons who smoke cigarette will be same

Let's say:
In low-nicotine cigarettes, the quantity per cigarette is 1 unit. Therefore in a pack of 10, total nicotine is 10 units.
In high-nicotine cigarettes, the quantity per cigarette is 2 unit. Therefore in a pack of 10, total nicotine is 20 units.

Now, according to conclusion, the nicotine levels in blood of smokers who smoke one pack of low or high nicotine cigarettes are identical.

'A' says that blood can not absorb more nicotine than the levels present in lowest nicotine level cigarettes i.e. 10 units, In our case b'cos we just have 2 types. High and Low. It is the cigarette with lower nicotine level

HTH