There is nothing ambiguous ___ all illegal immigrants should be sent back across the border immediately.
A:that
B:which
C:whether
D:when
读等高线图,图中可以发育为河流的是( )
如图,△AOB中,∠AOB=90°,AO=2,BO=4,△AOB绕顶点O逆时针旋转到△A′OB′处,此时线段A′B′与BO的交点E为BO的中点,则线段B′E的长度为____
和 各代表什么数?请你算一算。
30÷ =6 + + =17 =____ =____