How to send event from activity to activity with Otto?

I have Three Activities. A, B, C.

This Process is A -> B -> C. They Share an Object. 'A' activity has adapter and gridView, gridView's child is that Object.

Click an Object go to B activity, B activity is show the Object's data. C activity is EditMode for the Object.( if Click a button, then send data to A activity.)

How to send Event or data from C to A.