4. The hotel was noisy. They didn't go to their party.

-> A. As the hotel was noisy, they didn't go to their party.

5. The noisy hotel was very busy. She couldn't answer the phone.

-> C. The noisy hotel was so busy that she couldn't answer the phone.

6. Tim graduated with a good degree. He joined the ranks of the unemployed.

-> A. Tim graduated with a good degree. He would join the ranks of the unemployed.

7. Smallpox is a dangerous disease. Malaria is a dangerous disease.

-> A. Smallpox is a dangerous disease, and so is malaria.

8. The substance is very toxic. Protective clothing must be worn at all times.

-> B. So toxic is the substance that protective clothing must be worn at all times. 
9. Although she was very old, she looked very grateful.

-> C. Despite her old age, she looked very grateful.

10. Though he tried hard, he didn't succeed.

-> B. However hard he tried, he didn't succeed.

11. Slippery as the roads were, they managed to complete the race.

-> B. The roads were slippery but they managed to complete the race.

12. Seeing that he was angry, she left the office.

-> D. She left the office when she saw how angry he was.