(a) [for the mathematically-keen only! ] show that in a matter-dominated open 0, the angular diameter distance to an object at redshift universe with a z is given by diam - 2c

Which of the following situations is impossible? a) an object has velocity directed east and acceleration directed east. b) an object has zero velocity but non-zero acceleration. c) an object has constant non-zero velocity and changing acceleration. d) an object has velocity directed east and acceleration directed west. e) an object has constant non-zero acceleration and changing velocity.