填写合适的单位:
(1)一棵大树高5______.
(2)一个火柴盒的体积是21______.
(3)一间教室面积是50______.
(4)一袋牛奶的体积约235______.
(5)一支钢笔长约14______.
(6)一张床的面积约3______.