In order to have non stop flight a plane has to be given fuel in the middle of flight,Which of these is necessary for mid air refuelling to happen?

Question

$\begin{array}{1 1} (a)\;\text{One plane should be smaller than the other} \\ (b)\; \text{The plane should be flying above the clouds} \\ (c)\; \text{Planes should be flying as fast as each other} \\ (d)\; \text{One plane should be flying upwards and the other downwards} \end{array}$