Which of the following is the strongest Lewis base?

Which of the following is the strongest Lewis acid?

When a satellite moves around the earth in a certain orbit,the quantity which remains constant is .........

$A$ body of mass $M \ kg$ is at the top point of a smooth hemisphere of radius $5 \ m$. It is released to slide down the surface of the hemisphere. It leaves the surface when its velocity is $5 \ m/s$. At this instant,the angle made by the radius vector of the body with the vertical is: (Acceleration due to gravity $g = 10 \ m/s^2$) (in $^{\circ}$)

The area (in square units) of the region bounded by $x^2=8y$,$x=4$ and the $X$-axis is:

$X + H_2SO_4 \to Y$ (a colorless gas with a pungent smell).
$Y + K_2Cr_2O_7 + H_2SO_4 \to$ green colored solution.
What are $X$ and $Y$?