What is obtained by the reaction of $HCOOH$ with concentrated $H_2SO_4$?

In which of the following does aluminum become passive?

Identify $A$ and $B$ in the following reaction:
$A \xrightarrow[\Delta]{aq. NaOH} C_2H_5OH \xleftarrow{AgOH} B$

Which of the following metals becomes passive towards concentrated $HNO_3$?

The objective and eyepiece of an astronomical telescope are double convex lenses with refractive index $1.5$. When the telescope is adjusted to infinity,the separation between the two lenses is $16 ~cm$. If the space between the lenses is now filled with water (refractive index $4/3$) and the telescope is again adjusted for infinity,then the present separation between the lenses is: (in $~cm$)

$A$ particle undergoes uniform circular motion. About which point on the plane of the circle will the angular momentum of the particle remain conserved?