Explanation:

- B. Penicillin is one of the most useful drugs man has invented.

- This introduces the main topic: penicillin and its significance as a remarkable invention.

- A. This invention has helped us to heal wounds caused by bacteria.

- This sentence provides specific information about the utility of penicillin, linking it to the healing of bacterial wounds.

- D. Its use has saved the lives of hundreds of thousands of soldiers.

- This elaborates on the impact of penicillin, highlighting its role in saving many soldiers' lives, which is a historical reference to its use in wars.

- C. In the beginning however, very few people knew of this discovery and its use.

- This sentence provides historical context, explaining that initially, its use and discovery were not widely known.

Option 3, BADC, forms a logical sequence for the narrative about penicillin.