一座拦河坝的横截面是个梯形,它的面积是72平方米,它的上底是12米,下底是18米,这座拦河坝的高是____米?
某同学摇动苹果树,从同一高度有一个苹果和一片树叶同时从静止落下,发现苹果先落地,下列说法正确的是( )
13-6=____
13-7=____
11-9=____
11-2=____
13-9=____
13-4=____
12-7=____
12-5=____
—Do you know _________ boy under the tree?
— Yes, he knows a lot about dinosaurs though he is _________ 11-year-old boy.