Acid strength decreases in the series: hcl > hso4– > hcn. which of these species is the strongest base?

Answer : The acid strength of HCl is strongest among the given series.
So, the strength decreases as per this trend HCl > > HCN;

So when identifying the weakest base it has to be CN as it has less acid strength as compared to the series as it acts as a lewis base which donates its pair of electrons. They are weak bases generally but when compared with acids they are considered to be the strongest. 

Although the diagram is not shown, i know that the convection is heated by a the core.

The most plausible answer is b, duration. the given amount of time is considered how long it last for.