Which event of the ' action potential ' is indicated by the letter (A) in the diagram ?

Question

$\begin {array} {1 1} (1)\;Influx\: of\: K^+ & \quad (2)\;Efflux\: of \: Na^+ \\ (3)\;Influx\: of\: Na^+ & \quad (4)\;Efflux\: of \: K^+ \end {array}$