Which one of the following pairs of molecules will have permanent dipole moments for both members

Which one of the following compounds has the smallest bond angle in its molecule?

An ethar is more volatile than an alcohol having the same molecular formula. This is due to

A function $$f$$ from the set of natural numbers to integers defined by $$$f\left( n \right) = \left\{ {\matrix{ {{{n - 1} \over 2},\,when\,n\,is\,odd} \cr { - {n \over 2},\,when\,n\,is\,even} \cr } } \right.$$$ is