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When a paragraph has unity, all the supporting sentences discuss only one idea. From beginning to end, each sentence is directly related to the topic. In some languages, it is acceptable to wander away from the topic - to make little side trips to ideas that are somewhat, but not directly, related to the main topic. In English, doing so is not acceptable because it breaks the unity of the paragraph.
My first apartment was very small. It was a studio apartment, so it had only one main room and a bathroom. The main room was divided into three areas. At one end of it was a kitchenette, where I cooked and ate my meals. My living/sleeping area was at the opposite end. I had just enough space for a bed, a coffee table, a floor lamp, and a small television. My study area was against the back wall. I lived there for two years, but I moved because my landlord raised the rent. My apartment was so small that I could never invite more than three friends over at the same time.