Which of the following sequences is not an $A.P.$?

The first term of an $A.P.$ is $a$ and its common difference is $d$. Then,the $18^{th}$ term of the $A.P.$ is..........

For the finite $A.P.$ $3, 8, 13, \ldots, 253,$ find the $20^{th}$ term from the end.

If the first term of an $AP$ is $-5$ and the common difference is $2,$ then the sum of the first $6$ terms is

The taxi fare after each $km$,when the fare is $Rs. 15$ for the first $km$ and $Rs. 8$ for each additional $km$,does not form an $AP$ as the total fare (in $Rs.$) after each $km$ is $15, 8, 8, 8, \ldots$. Is the statement true? Give reasons.

In which of the following situations,do the lists of numbers involved form an $AP$? Give reasons for your answers.
$(i)$ The fee charged from a student every month by a school for the whole session,when the monthly fee is $Rs. 400$.
$(ii)$ The fee charged every month by a school from Classes $I$ to $XII$,when the monthly fee for Class $I$ is $Rs. 250$,and it increases by $Rs. 50$ for the next higher class.
$(iii)$ The amount of money in the account of Varun at the end of every year when $Rs. 1000$ is deposited at simple interest of $10\%$ per annum.
$(iv)$ The number of bacteria in a certain food item after each second,when they double in every second.