A pipe $A$ is connected with other pipes $B$ and $C$ as shown in the figure.The areas of cross-section of $A, B$ and $C$ are respectively $\alpha, \frac{\alpha}{2}$ and $\frac{\alpha}{4}$ .If the velocities of flow of water through $A$ and $B$ are $10 \mathrm{~m} / \mathrm{sec}$ and $6 \mathrm{~m} / \mathrm{sec}$ ,respectively,then velocity of flow,$V_c$ along $C$ is

$21 \mathrm{~m} / \mathrm{sec}$
$12 \mathrm{~m} / \mathrm{sec}$
$28 \mathrm{~m} / \mathrm{sec}$
$18 \mathrm{~m} / \mathrm{sec}$
A particle of mass $m$ is suspended from a point O by a string of length $R$ .It is given a velocity $u=3 \sqrt{g R}$ at the bottom.The difference in tension at point $B$ and at the point $C$ is $m$ 
$6 mg$
$3 mg$
$4 m g$
$8 m g$
The de-Broglie wavelength of an electron in 4th orbit is (where $r=$ radius of the 1st orbit)
$2 \pi r$
$4 \pi r$
$8 \pi r$
$16 \pi r$
An electromagnetic wave,whose wave normal makes an angle of $45^{\circ}$ with the vertical,is travelling in air and strikes a horizontal liquid surface.While travelling through the liquid,it gets deviated by $15^{\circ}$ .If the speed of electromagnetic wave in air is $3 \times 10^8 \mathrm{~m} / \mathrm{s}$ ,then the speed of electromagnetic wave in the liquid will be
$\frac{\sqrt{2}}{3} \times 10^8 \mathrm{~m} / \mathrm{s}$
$1.5 \times 10^8 \mathrm{~m} / \mathrm{s}$
$2.1 \times 10^8 \mathrm{~m} / \mathrm{s}$
$2.5 \times 10^8 \mathrm{~m} / \mathrm{s}$
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