多年来，非洲人口的突出特点是（ ）

根据句意和汉语提示，在空格中填入正确的单词 。
⑴North points up on a ____  (地图).
⑵There is a tall tree____  (在。。。。。。的后面) the teaching building.
⑶____ (没有什么) is difficult to the men who will try.
⑷____  (十月) 1st is the National day of the PRC.
⑸He always ____ (抄写) new words on pieces of paper and reads them aloud.
⑹It's____ (礼貌 ) to be quiet in the library.
⑺No one can____ (成功) without any hard work.

若圆中一段弧长正好等于该圆外切正三角形的边长，设这段弧所对的圆心角是$\theta$，则$\sin \theta$的值所在的区间为（   ）

一个物体做匀速直线运动，全程是24m，前一半路程用时4s，后一半路程平均速度为2m/s，求：

如图，在矩形ABCD中，对角线AC，BD相交于点O，若∠AOD=2∠AOB，AB=4cm，则矩形ABCD的面积是____cm²．