Here are little cars that may some day take the place of today’s cars. If everyone drives such a car in the future, there will be less pollution in the air. There will also be more parking spaces in cities, and the streets will be less crowded. Three such Cars fit in the space now needed for one Car of the usual size. The little cars will cost much less to own and to drive. Driving will be safer, too, as these little cars can go only 55 miles per hour. The Cars of the future will be fine for getting around a city, but they will not be useful for long trips.

What’s the disadvantage for little cars
They will not be useful for（）

让患者步行6min或12min，记录其所能行走的最长距离。试验与上述分级运动试验有良好相关性，对于不能进行活动平板运动试验的患者可行6min或12min行走距离测定，以判断患者的运动能力及运动中发生低氧血症的可能性。采用定距离行走，计算行走时间，也可以作为评定方式的试验是（）

A.极量运动试验

B.低水平运动试验

C.症状限制性运动试验

D.定量行走试验

E.呼吸气分析