why at the resonance frequency the impedance of the circuit reduces to R, i.e. why the LC part of the circuit appears to be invisible? (LCR circuit in series).
how accurately can resonant RLC circuit discriminate between different driving frequencies?
I think you meant to say the selectivity is dependant on Q. So high Q means high selectivity. This is probably too much information, but have a look here: RLC circuit - Wikipedia, the free encyclopedia