Taking the Earth to be a metallic sphere,its capacity will approximately be

The stratosphere acts as a conducting layer for the Earth. If the stratosphere extends up to $50 \ km$ from the Earth's surface,calculate the capacitance of the spherical capacitor formed between the Earth's surface and the stratosphere in $F$. Take the radius of the Earth as $6400 \ km$.

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ Two metallic spheres are charged to the same potential. One of them is hollow and another is solid,and both have the same radii. The solid sphere will have a lower charge than the hollow one.
Reason $R:$ Capacitance of metallic spheres depends on the radii of the spheres.
In the light of the above statements,choose the correct answer from the options given below.

Two charged parallel plate capacitors,each with separation between plates equal to $d$,are separated by a large distance $L >> d$. Then the force of interaction between them is proportional to

The radius of a metallic sphere if its capacitance is $1/9\,F$,is

Eight drops of mercury, each of same radius and same charge, combine to form a bigger drop. The ratio of the capacitance of the bigger drop to that of each smaller drop is: (in $ : 1$)