Which of the following is the pair of biofertilizers?

If $f: R \rightarrow R$ is defined by $f(x)=x-[x]-\frac{1}{2}$ for $x \in R$,where $[x]$ is the greatest integer not exceeding $x$,then $\{x \in R: f(x)=\frac{1}{2}\}$ is equal to :

In a $\triangle ABC$,the expression $\frac{\cos C+\cos A}{c+a}+\frac{\cos B}{b}$ is equal to

$Fe_2Cr_2O_4 + Na_2CO_3 + O_2$ $\xrightarrow{\text{Fusion}} [X]$ $\xrightarrow{H^+ / H_2O} [Y]$ $\xrightarrow{H_2O_2 / H^+} [Z]$
Which of the following statements is true for the compounds $[X], [Y],$ and $[Z]$?

Which one of the following is more readily hydrolysed by $S_{N}1$ mechanism?

The value of $k$ such that the lines $2x - 3y + k = 0$,$3x - 4y - 13 = 0$,and $8x - 11y - 33 = 0$ are concurrent,is