Q. In Bottom-up integration

(A) The low-level components of a system are integrated and tested before the higher-level components have been developed

(B) The testing process starts with modules at lower levels in the hierarchy and works up the hierarchy of modules until the final module is tested.

(C) An event driven control system, where an event is broadcasted to each sub- system and any sub-system that can handle that event responds to it.

(D) Control starts of the top of a sub routine hierarchy and through sub routine calls passes to lower levels.