Which sentence is punctuated correctly? i want to buy the new jacket but it is too expensive. i want to buy the new jacket, but it is too expensive. i want to buy the new jacket; but it is too expensive. i want to buy the new jacket, it is too expensive

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.