Which of the following oxides cannot act as a reducing agent?

Which of the following is not a Lewis acid?

$A$ particle of mass $m$ is constrained to move on the $x$-axis. $A$ force $F$ acts on the particle. $F$ always points toward the position labeled $E$. For example,when the particle is to the left of $E$,$F$ points to the right. The magnitude of $F$ is constant except at point $E$ where it is zero. The system is horizontal. $F$ is the net force acting on the particle. The particle is displaced a distance $A$ towards the left from the equilibrium position $E$ and released from rest at $t=0$. What is the period of the motion?

In an electrochemicalcell, dichromate ions in aqueous acidic medium are reduced to $Cr ^{3+}$. The current $($in amperes$)$ that flows through the cell for $48.25$ minutes to produce $1$ mole of $Cr ^{3+}$ is $....$ . Use: $1$ Faraday $=96500 C mol ^{-1}$

What is the vital capacity of our lungs?

In a $\triangle ABC$,if $3a = b + c$,then $\cot \frac{B}{2} \cot \frac{C}{2}$ is equal to :