Which of these number is main idea? i need !

According to eric shlosser’s introduction to fast food nation what is his book primarily about?

The greatest gift the sumerians gave the world was the invention of writing. the sumerians were wealthy people. they needed some way to keep track of what they owned. they began drawing pictures. they used a reed as a pen. they drew on soft pieces of clay. the soft clay was then dried in the sun. the tablet became a permanent record. later, the sumerian drawings changed into wedge-shaped symbols. this kind of writing is called cuneiform. by putting symbols together, the sumerians could write entire sentences.

What is the source of tension in this excerpt? ellery searching everything insects filling the light fixtures ellery standing on the desks louise carpenter whispering