Selection is a method related to which of the following?

The angle between the lines whose direction cosines are given by the equations $l^2+m^2-n^2=0$ and $l+m+n=0$ is

The partial pressure of oxygen in the alveoli of the lungs is

$A$ ray of light is incident on the surface of a glass plate at an angle of incidence equal to Brewster's angle $\phi$. If $\mu$ represents the refractive index of glass with respect to air,then the angle between the reflected and refracted rays is:

If $f: R \rightarrow R$ is defined by $f(x)=x-[x]-\frac{1}{2}$ for $x \in R$,where $[x]$ is the greatest integer not exceeding $x$,then $\{x \in R: f(x)=\frac{1}{2}\}$ is equal to :

The number of solutions for the equation $x^2 - 5|x| + 6 = 0$ is