That river is too* wide to swim across.

Here are the completed sentences:

• 21. That river is too wide to swim across.

  • "Too" indicates an excessive degree, making it impossible or undesirable to swim across.
  • The correct option is D. too.

• 22. He called a few girls to help in the kitchen.

  • "A few" is used with countable nouns (girls) to mean a small, but sufficient, number.
  • The correct option is A. a few.

• 23. I have known Mr. Titus for many years.

  • The phrase "I have" requires the past participle form of the verb, which for "know" is "known", forming the present perfect tense.
  • The correct option is C. known.

• 24. We are happy to see an ostrich dancing.

  • "An" is used before words that start with a vowel sound, such as "ostrich".
  • The correct option is B. an.

• 25. Work hard or else you will fail your exams.

  • "Or else" is a common conjunction used to introduce a negative consequence if the preceding action is not taken.
  • The correct option is D. or.

• 26. Brian has a new bike. It belongs to him.

  • "Him" is the objective pronoun used to refer to a male person (Brian) as the recipient of the action or the object of a preposition.
  • The correct option is C. him.

• 27. You ate more bananas than me.

  • "More" is used for comparison with both countable (bananas) and uncountable nouns.
  • The correct option is A. more.

• 28. The dentist checked all her teeth.

  • "Teeth" is the irregular plural form of "tooth".
  • The correct option is B. teeth.

• 29. They walked carefully because the floor was slippery.

  • "Carefully" is an adverb that describes how they walked, indicating caution due to the slippery floor.
  • The correct option is D. carefully.

• 30. Nobody heard the burglars break into the shop.

  • "Nobody" is an indefinite pronoun meaning "no person", which fits the context of no one hearing the burglars.
  • The correct option is C. Nobody.

Here are the answers: 21. $\boxed{\text{D. too}}$ 22. $\boxed{\text{A. a few}}$ 23. $\boxed{\text{C. known}}$ 24. $\boxed{\text{B. an}}$ 25. $\boxed{\text{D. or}}$ 26. $\boxed{\text{C. him}}$ 27. $\boxed{\text{A. more}}$ 28. $\boxed{\text{B. teeth}}$ 29. $\boxed{\text{D. carefully}}$ 30. $\boxed{\text{C. Nobody}}$