Which of the following will release hydrogen gas upon reaction with hydrochloric acid?

For a gas,$\frac{R}{C_{V}}=0.4$,where $R$ is the universal gas constant and $C_{V}$ is the molar specific heat at constant volume. The gas is made up of molecules which are

An eight-digit number divisible by $9$ is to be formed using digits from $0$ to $9$ without repeating the digits. The number of ways in which this can be done is: (in $(7!)$)

In the hardening stage of plaster of Paris,the compound formed is:

The differential equation of all circles which pass through the origin and whose centre lies on the $y$-axis is

$A$ Zener diode,having a breakdown voltage equal to $15\, V$,is used in a voltage regulator circuit as shown in the figure. The current through the Zener diode is.....$mA$.