In English, when we want to express a preference for one option over another, we use "to" to indicate that preference. For example: - "I prefer tea to coffee." (I like tea more than coffee.) - "She prefers reading to watching TV." (She likes reading more than watching TV.) Similarly, in the sentence "He preferred travelling by road to travelling by air," the word "to" is used to show that travelling by road is the preferred option compared to travelling by air. On the other hand, "than" is used in comparative structures to show a difference between two things or actions. For example: - "Tea is better than coffee." (Tea and coffee are being compared, and tea is considered better in this comparison.) - "She runs faster than him." (Her running speed is compared to his, and she runs faster in this comparison.)