Which of the following elements never acts as a reducing agent in a chemical reaction?

The main reason for the brown ring test of $NO_3^-$ is:

Which of the following substances can act as both an oxidizing agent and a reducing agent?

Identify the reducing agent in the following reaction:
$H_2O_{2(aq)} + ClO_{4(aq)}^{-} \rightarrow ClO_{2(aq)}^{-} + O_{2(g)}$

Consider the following reaction:
$BrCH_2CH_2F + SbF_5 \xrightarrow{SO_2, -60^{\circ}C} CH_2-CH_2-Br^+ + SbF_6^-$
In this reaction,$SbF_5$ acts as:

The oxide which cannot act as a reducing agent is