Final answer:
Cohesion in writing is achieved through the use of transitional words and phrases, while unity is achieved by having a single, clear main idea.
Step-by-step explanation:
In order to achieve cohesion in a piece of writing, it is important to employ transitional words and phrases. These transitions help to connect ideas and create a smooth flow between sentences and paragraphs. By using transitions, the writer is able to guide the reader through the different sections of the writing and ensure that the information is organized and easy to follow.
On the other hand, unity in writing is achieved by using a single, clear main idea. This main idea acts as the central focus of the writing and helps to keep the content focused and coherent. By avoiding conflicting viewpoints and irrelevant information, the writer can ensure that the writing maintains unity and stays on topic.