Which one is not correct? a comma can be used to:

separate two dependent clauses
separate an independent clause from a dependent clause
separate two independent clauses with a conjunction
separate a series of objects

If the author of the gift of the magi had made the reader aware of the gifts that jim had purchased in the beginning of the story this would be an example of

Compare: what are the similarities and differences between "artificial flavors" and "natural flavors"? why does schlosser explain these two terms in such detail?

The greatest gift the sumerians gave the world was the invention of writing. the sumerians were a 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.