How is the moon similar to a dollar?

There was once a college that offered a class on probability applied to the real world. The class was relatively easy, but there was a catch. There were no homework assignments or tests, but there was a final exam that would have only one question on it. When everyone received the test paper it was a blank sheet of paper with a solitary question on it: 'What is the risk?'.Most students were able to pass, but only one student received 100% for the class! Even stranger was that he only wrote down one word!
What did he write?

What has one eye, but can’t see?

Can you solve the below algebraic mathematical equation?

(J+O+I+N+T)3 = JOINT

Which city is three-fifths chick two third cat and 50% goat?

What has many teeth, but cannot bite?

What does Alexander the Great and Winnie the Pooh have in common?

What can you serve, but never eat?

A sea diver is a real show-off. He showed everyone that he can hold his breath underwater for 15 minutes.

I went to the diver and told him that I can be underwater for double the time i.e 30 minutes.
He responded that he will give me 100$ if I would be able to do it. I won 100$.

What did I do?

What is the coolest letter in the alphabet?