Which one of the following removes temporary hardness of water?

The suspension of slaked lime in water is known as

$A$ $Ge$ specimen is doped with $Al$. The concentration of acceptor atoms is $\sim 10^{21} \text{ atoms}/m^3$. Given that the intrinsic concentration of electron-hole pairs is $\sim 10^{19} /m^3$,the concentration of electrons in the specimen is:

The sum of the fourth powers of the roots of the equation $x^3+x+1=0$ is

Helium mixed with oxygen is used in the treatment of

Which of the following is a valid mathematical statement?