What does orthoboric acid contain?

In which of the following reactions,chlorine acts as an oxidising agent?
$(i)$ $CH_3CH_2OH + Cl_2 \longrightarrow CH_3CHO + HCl$
$(ii)$ $CH_3CHO + Cl_2 \longrightarrow CCl_3CHO + HCl$
$(iii)$ $CH_4 + Cl_2 \xrightarrow{hv} CH_3Cl + HCl$
The correct answer is

$A$ narrow tube is bent in the form of a circle of radius $R,$ as shown in the figure. Two small holes $S$ and $D$ are made in the tube at positions right-angled to each other. $A$ source placed at $S$ generates a wave of intensity $I_0$ which is equally divided into two parts: one part travels along the longer path,while the other travels along the shorter path. Both the waves meet at the point $D$ where a detector is placed. The maximum value of $\lambda$ to produce a maxima at $D$ is given by

Two soap bubbles of radii $r_1 = 4 \, cm$ and $r_2 = 5 \, cm$ are touching each other over a common surface $S_1S_2$ (as shown in the figure). The radius of curvature of this common surface will be ....... $cm$.

What is the use of potassium dichromate $(K_2Cr_2O_7)$?

If $x$ is a solution of the equation $\sqrt{2x+1} - \sqrt{2x-1} = 1$ where $x \geq \frac{1}{2}$,then $\sqrt{4x^2-1}$ is equal to