The method of concentrating the ore which makes use of the difference in density between ore and impurities is called

If $\theta$ is the angle between the tangents from $(-1,0)$ to the circle $x^2+y^2-5x+4y-2=0$,then $\theta$ is equal to

Assertion $(A)$: Equal moles of different substances contain the same number of constituent particles.
Reason $(R)$: Equal weights of different substances contain the same number of constituent particles.
The correct answer is

From which plant did $D.J. Ivanowsky$ discover viruses?

If $A$ is a non-zero square matrix of order $n$ with $\operatorname{det}(I+A) \neq 0$ and $A^3=O$,where $I$ and $O$ are the unit and null matrices of order $n \times n$ respectively,then $(I+A)^{-1}$ is equal to

$A$ $Ge$ 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: