Which of the following species can act as a reducing agent?

The power factor of the circuit as shown in the figure is:

$A$ convex lens of focal length $F$ produces a real image $n$ times the size of the object. The image distance is

The image of the point $(3, 2, 1)$ in the plane $2x - y + 3z = 7$ is

Wires $A$ and $B$ are made from the same material. $A$ has twice the diameter and three times the length of $B$. If the elastic limits are not reached,when each is stretched by the same tension,the ratio of energy stored in $A$ to that in $B$ is

If the harmonic mean between the roots of $(5+\sqrt{2}) x^2-b x+(8+2 \sqrt{5})=0$ is $4$,then the value of $b$ is