The test was so long that we couldn't finish it in an hour

1. The test was so long that we couldn't finish it in an hour.
-> Since
2. Doing exercises regularly will make us healthier.
-> If
3. "Why don't you stop working and take a rest, Peter ?"
-> He suggested
4. He designed bags cleverly.
-> They
5. Students achieve good results because of learning the lessons carefully.
-> Because
6. We can't go fishing in this lake because the water is highly polluted.
-> Because of