Fill in the gap with can, could, be able to

I. Fill in the gap with can, could, be able to.

1. _________ you stand on your head?   - I ___________ when I was at school but now I __________.

2. When I was a child I __________ understand adults but now I am an adults I ________ not understand children.

3. When the fog lifts we __________ see where we are.

4. I used to  _____________ stand on my head but I __________ not do it now.

5. The swimmer was very tired but he __________ reach the shore before he collapsed.