We can . . . . . . . . to travel by air, but we . . . . . . . . train journey.

The correct answer is: C. afford, prefer.

We can afford to travel by air, but we prefer to take a train journey. This is because air travel is often more expensive than train travel, and it can also be more stressful and time-consuming. Train travel is a more relaxing and enjoyable way to travel, and it can also be a more environmentally friendly option.

Here is a brief explanation of each option:

• A. plan, enjoy: This is not the correct answer because we can plan to travel by air, but we may not enjoy it.
• B. aspire, commute: This is not the correct answer because we can aspire to travel by air, but we may not be able to afford it.
• C. afford, prefer: This is the correct answer because we can afford to travel by air, but we prefer to take a train journey.
• D. decide, undertook: This is not the correct answer because we can decide to travel by air, but we may not undertake the journey.
• E. decline, accepted: This is not the correct answer because we can decline to travel by air, but we may accept a train journey.