Who was the first scientist to use the term 'virus' as a pathogen of infectious diseases?

Two closed organ pipes of length $100\,cm$ and $101\,cm$ long give $16$ beats in $20\,sec$ when each pipe is sounded in fundamental mode. Calculate the velocity of sound in $m/s$.

The solution of $\frac{dy}{dx} = \frac{y^2}{xy - x^2}$ is

$Mg$ forms $Mg^{2+}$ but not $Mg^+$. The reason is ........

The major product of the following reaction is

In which of the following molecules is the $N$ atom not $sp^2$ hybridized?