Let's find the antonyms for each sentence:

1. I did attend the lecture and the professor held forth about economic theory.
- Antonym: A. mentioned nothing (implying not attending or not speaking about anything)

2. I don't think the police's theory will hold water. The suspect has an alibi.
- Antonym: B. be supported (indicating the theory being disproved or not being valid)

3. The patient's condition is improving by degrees.
- Antonym: A. marginally (suggesting minimal improvement or worsening)

4. John seems unable to keep a civil tongue.
- Antonym: C. speak to others in an impolite way (indicating speaking politely)

5. Egyptians are trying to prevent their historical monuments from succumbing to the ravages of time.
- Antonym: B. yielding to (suggesting giving in or allowing something to happen)

So, the antonyms are:
1. A. mentioned nothing
2. B. be supported
3. A. marginally
4. C. speak to others in an impolite way
5. B. yielding to