Identify the number of each sentence that contains the topic idea. 1. Generation gap refers to the diatance and contradiction between the old and the young. 2. It's a common phenomenon that exits everywhere in the world and influences both the old and the young. 3. Generation gap results from different understanding and appreciation of new things, and different attitudes to traditional principles and beliefs. 4.Therefore, we say where are the old and the young, there is a generation gap.