Trouble With Triangles with Altitude

Okay. I'd doing this assignment and I came across a problem I absolutly know i got right but the computer says otherwise (possibly a programming error). Here it is:

In the diagram of ΔABC, m∠ACB = 90 and is an altitude. If AD = 2 and DB = 6, find AC.

http://castlelearning.com/review/Cou...ry/MJ19015.gif

1.) http://castlelearning.com/review/Cou...ry/sqrt-12.gif

2.) 4http://castlelearning.com/review/Cou...try/sqrt-3.gif

3.) 6

4.) 4

It said it was 4.) 4, but I disagree.