In the mechanism ABCD shown in the given figure, the fixed link is denoted as (1), crank AB as (2) rocker BD (3), Swivel trunnion at C as (4) the instantaneous centre $I_{41}$ is at
(a) The centre of swivel trunnion
(b) The intersection of line AB and a perpendicular to BD to
(c) Infinity along AC
(d) Infinity perpendicular to BD