Respiration in Organisms

A food stall owner was preparing dough for making bhaturas.
He added a pinch of yeast and sugar to the dough and left it in
a warm place. After few hours, the dough had risen. There was
a sour smell too.
(a) Why did the dough rise?
(b) Why did the dough smell sour?
(c) Why was sugar added to the dough?
(d) What would have happened if the dough was kept in the
refrigerator, soon after it was prepared?