A coil has an inductance of 2.5 H and a resistance of 0.5 r. If the coil is suddenly connected across a 6.0 volt battery, then the time required for the current to rise 0.63 of its final value is
(1) 3.5 sec
(2) 4.0 sec
(3) 4.5 sec
(4) 5.0 sec