Fill in the appropriate units in the horizontal lines: the length of a piece of chalk is about 1 _ _ _ _ _ _ _ _ _ _ The thickness of a math book is 8 _ _ _ _ _ _ _ _ The height of a big tree is about 10 _ _ _ _ _. Solution: The length of a piece of chalk is about 1 decimeter, the thickness of a math book is 8 mm, the height of a big tree is about 10 meter, and Xiaoming sleeps for about 9 hours every day, so the answers are decimeter, millimeter, meter and hour.