a retractable wheels的用法和样例：
- A retractable steering column is also fitted.
- Paws might be more useful than wheels in some situations, but cats'claws are retractable: why not retractable wheels?
- A retractable handle slides in-and-out for easy towing of your bag and heavy-duty wheels are located on the opposite side for effortless ease of towing as well.
- The house has a retractable staircase that lowers to the ground.
- a retractable landing wheel [gear]