1. Paul likes to help the sick, therefore/so he has donated blood several times. (it's obvious)
2. Take a coat because/as/since it’s cold outside. (explanation)
3. Homeless children are ... because/as/since they don’t have enough food, warm clothes and schooling. (explanation)
4. Nick went to bed early because/since/as he felt very tired. explanation
5. We went to the nursing home and we talked and sang for the elderly. (two different sentences, linked with and)