Progressive matrix question - squares, circles, triangles in the corners

This question comes from www.test-iq.org’s free demo IQ test. I took the extremely short test just for fun. I had no problem in doing the other questions, but I have problem in doing this question:

I did find out a pattern in the shapes in the squares, but I did not find out a pattern that governs where the shapes are in the square. Any help in this problem would be appreciated. Thank you.