In an $\mathrm{AP}$, if $a=1$, $a_n=20$ and $S_n=399$, then $n$ is equal to 

Which of the following form of an $\mathrm{AP}$? Justify your answer.

$-1, -1, -1, -1,...$

Which of the following form of an $\mathrm{AP}$? Justify your answer.

$0, 2, 0, 2, ...$

Which of the following form of an $\mathrm{AP}$? Justify your answer.

$1, 1, 2, 2, 3, 3, ...$

Is the sequence $11, 22, 33, ...$ form of an $\mathrm{AP}$? Justify your answer.

Which of the following form of an $\mathrm{AP}$? Justify your answer.

$\frac{1}{2}, \frac{1}{3}, \frac{1}{4}, ...$

Which of the following form of an $\mathrm{AP}$? Justify your answer.

$2, 2^2, 2^3, 2^4$

Identify whether the given term represents an $A.P$. Justify your answer.

$\sqrt{3}, \sqrt{12}, \sqrt{27}, \sqrt{48},...$