I.
1. He couldn't meet you because he was busy.
2. She went to bed early because she was sick.
3. We spent the whole day in the garden because the weather was lovely.
4. I can't play volleyball because I am short.
5. We can't swim in this part of the river because the water is highly polluted.
II.
1. She can't work hard because of being very old.
2. He likes her because of her beauty.
3. He broke the vase because of being careless.
4. He left school because of his hard life.
5. I went to bed early because of feeling tired.