Why there's high voltage at series rlc resonant frequency?

Answer 1

With a series RLC circuit the same current goes through all three components. The reactance of the capacitor and inductor are equal and opposite at the resonant frequency, so they cancel out and the supply voltage appears across the resistor.

This means that the current is at its maximum, but that current, flowing through the inductor and the capacitor, produces a voltage across each that is equal to the current times the reactance.

The voltage magnification is the 'Q factor', equal to the reactance divided by the resistance.