A:发育和繁殖
B:产生新个体
C:增加变异性
D:改变遗传性
与圆x2+y2=1及圆x2+y2-8x+12=0都相外切的圆的圆心在( )
— Would you like to go for a picnic?
— Yes, that would ____ (be) very nice.