He is a _____ boy, so he can't go to the park by himself.
A：six-year-old
B：six year old
C：six-years-old
D：six years old
按要求，填单词。
small（对应词）____
这些书贵还是便宜?
Are these books expensive____？
蝙蝠通常在夜间出来活动、觅食，但它们从来不会撞击到墙壁、树枝上，并且能以很高的精确度确认目标．这主要是因为（ ）
