The image of an object approaching a convex mirror with a radius of curvature of 30 m along its optical axis is observed to move from 50.7 m to 35.6 m in 50 s. What is the average speed of the object?
a) 1.2 m/s
b) 1.8 m/s
c) 2.4 m/s
d) 3.0 m/s