I answered question 50 in the Applied Visual Design section on Learn How Bezier Curves Work, and my answer looks correct, but the program is telling me it is incorrect. Why? Does anyone know? I cannot move forward and pass this section until I can correct this ONE question… HELP!! Thanks-- 
Here is my answer:
#ball1 {
left: 27%;
animation-timing-function: cubic-bezier (0.25, 0.25, 0.75, 0.75);
}
#ball2 {
left: 56%;
animation-timing-function: ease-out;
}