1. Mary didn't go to school yesterday because she was sick.
=> Mary didn't go to school yesterday because of sickness
2. She went to bed early because she felt tired.
=> She went to bed early because of feeling tired
3. Ann didn't go to the circus with Betty because she had a bad cold.
=> Ann didn't go to the circus with Betty because of having a bad cold

4. John succeeded in his exam because he worked hard and methodically.

=> John succeeded in his exam because of working hard and methodically.
5. Margaret stayed home because her mother was sick.
=> Margaret stayed home because of her mother's sickness
6. I like him because his father is kind to me.
=> I like him because of the kindness of his father to me
7. I can't eat these fruits because they are green.
=> I can't eat these fruits because of the fact they are green.
8. I couldn't do the test because it was difficult.
=> I couldn't do the test because of the difficult test
9. I couldn't read the letter because it was dark.
=> I couldn't read the letter because of the fact that it was dark
10. He came late because it rained heavily.
=> He came late because of the heavy rain