问题描述：
It is very _____ for a person to know how fast a car is traveling.(横线填空）
六年级（第二学期）YLE P60页的选词填空题(understand，careful，fall down，difficult，within)
六年级（第二学期）YLE P60页的选词填空题(understand，careful，fall down，difficult，within)
问题解答：
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