How to find the length of the hypotenuse

Hi,

Sorry if this a easy problem.... I've been out of school now for 15 years and trying to study up on taking the ACT test to start college in August. There is so much I have forgotten. I could use some help with this problem. Incase you cannot read the question it says....

In the isosceles right triangle below, AB = 10 feet. What is the length, in feet, of AC ?

It has a multiple choice and I'm thinking the answer is E (I believe that this is what my brother said) but I need to know the steps of how he got that for the answer.

http://i241.photobucket.com/albums/f...htTriangle.jpg

Thanks a bunch for your help

Veronica