Question 25: C. My father explained the subject more clearly than my brother did.

• Explanation: Option C correctly uses "explained" (past tense) and "more clearly" to compare the clarity of the explanations.

Question 26: C. I am keen on going camping with my close friends in summer.

• Explanation: "Keen on" is the correct phrase to express interest in an activity. The other options contain incorrect prepositions or phrases.