A:豚鼠的棕色和黑色是一对相对性状
B:黑鼠是显性性状
C:子代黑鼠,有的是纯种,有的是杂种
D:虽然子代都是黑鼠,但他们都有棕色基因
已知椭圆x29+y2b2=1(0<b<3),左右焦点分别为F1,F2,过F1的直线交椭圆于A,B两点,若AF2+BF2的最大值为8,则b的值是( )
( )
I've asked him __________ us for dinner, but he said he was quite busy.