Which of the following is known as the pacemaker of the heart?

$A$ photoelectric cell is connected to a source of variable potential difference. The resulting photoelectric current $(\mu A)$ is plotted against the applied potential difference $(V)$. The graph with the broken line represents the data for a given frequency and intensity of the incident radiation. If the frequency is increased and the intensity is reduced, which of the following graphs (represented by unbroken lines) corresponds to the new situation?

An inclined plane makes an angle of $30^{\circ}$ with the horizontal. $A$ solid sphere rolling down this inclined plane from rest without slipping has a linear acceleration equal to:

The element with atomic number $12$ belongs to ......... group and ......... period.

If the lines $2x + 3y + 12 = 0$ and $x - y + k = 0$ are conjugate with respect to the parabola $y^2 = 8x$,then $k$ is equal to

If $\frac{x^2}{a} + \frac{2xy}{h} + \frac{y^2}{b} = 0$ represents a pair of straight lines such that the slope of one of the lines is twice the other,then $\frac{ab}{h^2} =$