In which of the following pairs of categories is it more difficult to determine the relationship to other taxa at the same level, thus making the problem of classification more complex?

Which one of the following reactions represents the oxidising property of $H_2O_2$?

If $f(x) = \frac{x^2}{x+a}$, then $f^{\prime \prime}(a)$ is equal to

Product $(A)$ is

In the reaction sequence,$C_2H_5Cl + KCN$ $\xrightarrow{C_2H_5OH} X$ $\xrightarrow[\Delta]{H_3O^+} Y$. What is the molecular formula of $Y$?

$A$ child is standing at one end of a long trolley moving with a speed $v$ on a smooth horizontal floor. If the child starts running towards the other end of the trolley with a speed $u$,the centre of mass of the system (trolley + child) will move with a speed.