Sure! Here are the completions for the sentences along with the explanations for each choice:

19. C. **among the richest people**
- This option is the most grammatically correct and commonly used phrase. "Among the richest people" correctly places Bill Gates in the context of being one of the richest individuals in the world.

20. C. **several top colleges**
- "Several" is the correct choice as it implies more than one but not all, making it appropriate for the context of Bill Gates being accepted at multiple prestigious universities without specifying exactly how many.

21. B. **relaxed**
- "Relaxed" fits well here because it suggests that he spent his time enjoying himself rather than stressing over his studies. The past tense "relaxed" indicates he engaged in this activity while he was at Harvard.

22. B. **because**
- "Because" provides the reason for leaving Harvard, which is that he wanted to open a software company. It connects the two clauses logically.

23. C. **is no longer**
- "Is no longer working" indicates that he has stopped working full-time for Microsoft. This phrase is the grammatically correct way to express that change in status.

24. B. **education**
- "Education" fits correctly in the context of what the foundation aims to promote. It refers to the system or process of educating people, which aligns with the mission of improving healthcare and reducing poverty.

So, the fully completed sentences would read:

Bill Gates is the co-founder of Microsoft, the world's largest PC software company. He is **(19) C. among the richest people** in the world. Bill Gates' interest in computers started when he was a teenager. After high school, he was accepted to **(20) C. several top colleges** in the USA: Harvard, Yale, and Princeton. He chose to go to Harvard to study law. At Harvard, he focused more on his interest in computers than on coursework. He often **(21) B. relaxed** by playing video games in Harvard's computer lab. He left Harvard before graduating **(22) B. because** he wanted to open a software company with his friend Paul Allen. However, he later said, "I don’t think dropping out is a good idea.’ In 2007, he received an honorary degree from Harvard. He began his speech by saying this to his father in the audience. "I have been waiting for more than 30 years to say this: Dad, I always told you I'd come back and get my degree.’ Bill Gates **(23) C. is no longer** working full time for Microsoft. He is now, with his wife, running The Bill & Melinda Gates Foundation which aims to improve healthcare, reduce poverty, and promote **(24) B. education** around the world.