Which of the following is an extinct human species?

If $A+2 B=\begin{bmatrix} 1 & 2 & 0 \\ 6 & -3 & 3 \\ -5 & 3 & 1 \end{bmatrix}$ and $2 A-B=\begin{bmatrix} 2 & -1 & 5 \\ 2 & -1 & 6 \\ 0 & 1 & 2 \end{bmatrix}$,then $\operatorname{Tr}(A)-\operatorname{Tr}(B) = $

Which of the following contains $p\pi - d\pi$ bonding?

In the following reaction,$A$ and $B$ respectively are:
$A$ $\xrightarrow{HBr} C_2H_5Br$ $\xrightarrow{B} A$

Which of the following is a linear molecule?

The variance of $20$ observations is $5$. If each observation is multiplied by $2$,then the new variance of the resulting observations is: