When two species of different genealogy come to resemble each other as a result of adaptation,the phenomenon is termed as

If the circle $x^2+y^2+6x-2y+k=0$ bisects the circumference of the circle $x^2+y^2+2x-6y-15=0$,then $k$ is equal to

An atomic power nuclear reactor can deliver $300\, MW$. The energy released due to fission of each nucleus of uranium atom $^{235}U$ is $170\, MeV$. The number of uranium atoms fissioned per hour will be

Two bodies of mass $4 ~kg$ and $5 ~kg$ are moving along east and north directions with velocities $5 ~m/s$ and $3 ~m/s$ respectively. The magnitude of the velocity of the centre of mass of the system is

$A$ wheel is rotating at $900 \ r.p.m.$ about its axis. When the power is cut-off,it comes to rest in $1 \ minute$. The angular retardation in $rad/s^2$ is

Select the correct option with respect to the age pyramids shown in the image.