The correct statements among the following are,

A. $\mathrm{Mo}(\mathrm{VI})$ and $\mathrm{W}(\mathrm{VI})$ are less stable than $\mathrm{Cr}(\mathrm{VI})$.

B. $\mathrm{Ce}^{4+}$ and $\mathrm{Tb}^{4+}$ are oxidant while $\mathrm{Eu}^{2+}$ and $\mathrm{Yb}^{2+}$ are reductant.

C. Cm and Am have seven unpaired electrons.

D. Actinoid contraction is greater from element to element than lanthanoid contraction.

Choose the correct answer from the options given below :

Correct statements from the following are

A. Potassium dichromate is an oxidising agent and it oxidises $\mathrm{FeSO}_4$ to $\mathrm{Fe}_2\left(\mathrm{SO}_4\right)_3$ in acidic medium.

B. Sodium dichromate can be used as primary standard in volumetric estimation.

C. $\mathrm{CrO}_4^{2-}$ and $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ are interconvertible in aqueous solution by varying the pH of the solution.

D. $\mathrm{Cr}-\mathrm{O}-\mathrm{Cr}$ bond angle in $\mathrm{Cr}_2 \mathrm{O}_7^{2-}$ is $126^{\circ}$.

Choose the correct answer from the options given below :

Among $Fe^{2+}$, $Fe^{3+}$, $Cr^{2+}$ and $Zn^{2+}$, the ion that shows positive borax bead test and with highest ionisation enthalpy is :

Which of the following sets includes all the species that will change the orange colour of $\mathrm{K_2Cr_2O_7}$ in acidic medium?