Which one of the following correctly represents the variation of electronegativity $(EN)$ with atomic number $(Z)$ of group $13$ elements?

$A$ car $P$ approaching a crossing at a speed of $10 \, m/s$ sounds a horn of frequency $700 \, Hz$ when $40 \, m$ in front of the crossing. The speed of sound in air is $340 \, m/s$. Another car $Q$ is at rest on a road which is perpendicular to the road on which car $P$ is reaching the crossing. The driver of car $Q$ hears the sound of the horn of car $P$ when he is $30 \, m$ in front of the crossing. The apparent frequency heard by the driver of car $Q$ is ...... $Hz$.

Let $a, b, c$ be positive integers such that $\frac{a \sqrt{2}+b}{b \sqrt{2}+c}$ is a rational number,then which of the following is always an integer?

Dispersive power depends on the following:

The two lenses of an achromatic doublet should have:

The slopes of the focal chords of the parabola $y^2=32x$,which are tangents to the circle $x^2+y^2=4$,are