Here are the most appropriate answers for each sentence:

18. All the witnesses **B. objected to** the groundless accusation of the court.
(Reason: "objected to" is the correct phrase to indicate disapproval of the accusation.)

19. Some countries have **A. refused** to sign in the environment treaty.
(Reason: "refused" fits well in the context of countries not agreeing to sign a treaty.)

20. Jim and Jenny **D. announced** that they were going to get married.
(Reason: "announced" is the correct word to indicate making a declaration about their marriage.)

So, the answers are:
18. B
19. A
20. D