For thousands of years comets have been a mystery to a man. Stephen Hawking
is a theoretical physicist. к двум предложениям задать к каждому по 5 вопросов(общий, специальный, альтернативный, вопрос к подлежащему, разделительный)

1. Have comets been a mystery to a man for thousands of years?  - общий
2. How long have comets been a mystery to a man? – специальный 
3. Have comets been a mystery to a man for thousands of years or for hundreeds of years? - альтернативный
4. What has been a mystery to a man for thousands of years? - к подлежащему
5. For thousands of years comets have been a mystery to a man, haven`t they? - разделительный

1. Is Stephen Hawking a theoretical physicist?
2. What is Stephen Hawking?
3. Is Stephen Hawking a theoretical physicist or a practical physicist?
4. Who is a theoretical physicist?
5. Stephen Hawking is a theoretical physicist, isn`t it? - разделительный