<span>d. There are four seasons in a year
The main idea is the topic of the certain literature.
</span><span>Because in an essay, there is a topic , all you have to do with the successive paragraphs you are writing is to write things related to the topic given. Every paragraph in the essay must be a subtopic to the overall topic therefore having another topic in each paragraph will make your essay incoherent and unparalleled in ideas perspective</span>
Anything which is not stable (unlikely to move) is unstable.
Sign language, writing/reading
Hope this helps!!