A:What colour is the flag of China?
B:What colour is the flag of Canada?
C:What colour is the U. K.'s flag?
如图,在Rt△OAB中,∠AOB=30°,将△OAB绕点O逆时针旋转100°得到△OA1B1,则∠A1OB=____ .
已知a→=(2,1)与b→=(1,2),要使|a→+tb→|最小,则实数t的值为____
既是气体的通道,又是食物的通道的是( )