When doing sit ups, is it better to exhale as you come up or fully exhale after !

When doing sit ups, is it better to exhale as you come up or fully exhale after your elbows touch your knees?

Is it better to hold your breathe while coming up and then exhaling fully when you sit upright or to gradually exhale as your are coming up?

I need to do 35 quick situps in one minute for a physical fitness test and need to know the proper breathing technique for best results.

Thanks to all who respond.

Exhale as you contract your muscles. In the case of sit ups it also allows for a better more complete contraction of the muscles.