It's easier to understand common nouns by contrasting them with proper nouns. Proper nouns are specific people, places, or things, while common nouns are more general people, places, or things. For example, "Hiroshima" is a proper noun, while "city" is a common noun. Based off this explanation, the common nouns in the above sentence are "morning," "bomber," "bomb," "city," "people," "city" (again), and "blast."