A:①-④
B:①—③
C:②—③
D:②—④
科学家将雌黑鼠乳腺细胞的细胞核移入白鼠去核的卵细胞内,待发育成早期胚胎后移入褐鼠的子宫内,该褐鼠产下的小鼠的体色与性别是( )
We can surely overcome these difficulties as long as we are________united.