Which of the following is the most stable ecosystem?

The transition from the state $n = 4$ to $n = 3$ in a hydrogen-like atom results in ultraviolet radiation. Infrared radiation will be obtained in the transition:

$A$ gramophone record is revolving with an angular velocity $\omega$. $A$ coin is placed at a distance $r$ from the centre of the record. The static coefficient of friction is $\mu$. The coin will revolve with the record if

In adiabatic compression,the decrease in volume is associated with

The coefficient of $x^n$ in $\frac{1-2x}{e^x}$ is:

$\int_{-2}^2 |[x]| \, dx$ is equal to