Convert the following Simple sentence into a Complex sentence: 'Being ill, he could not go to school.'

1. ক
   He was ill and could not go to school.
   
2. খ
   He could not go to school because he was ill.
   
3. গ
   As he was ill, he could not go to school.
   
4. ঘ
   He was ill, so he could not go to school.