Which sentence implies that something may not happen?
if i can sing well enough, i'll join the school chorus.
i didn't meet the requirements, so i can't apply for that scholarship.
why can't you go to the game tonight?
you have a tough decision to make.

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.