A pool is built in the shape of an ellipse, centered at the origin. The maximum vertical length is 40 feet, and the maximum horizontal width is 18 feet. Which of the following equations represents the pool?

x squared over 400 plus y squared over 81 equals 1
y squared over 400 plus x squared over 81 equals 1
x squared over 1600 plus y squared over 324 equals 1
y squared over 1600 plus x squared over 324 equals 1