Hey guys. I have no idea how to even start this question. Any ideas?? Thanks!

"A semi-circle is dawn outwardly on a chord 'AB' of a circle with center 'O' and units raidus. The point 'C' on this semi-circle which sticks out of the given circle the furthest is on the radius 'ODC' which is perpendcular to 'A'. Of course the Length of 'OC' is dependant on the choice of chord 'AB' in the first place. Determine 'AB' so that 'OC' is maximum"