科学家将灰鼠胚胎细胞的细胞核取出，移植到去除细胞核的黑鼠受精卵内，该受精卵在体外经短暂发育形成胚胎后，再移植到白鼠的子宫内完成胚胎发育直至幼鼠产出。下列对该实验的描述不正确的是

A．实验证明DNA是主要的遗传物质

B．实验证明细胞核中有遗传物质

C．白色雌鼠产出的小鼠体色为灰色

D．白色雌鼠提供胚胎发育场所

—What were you doing at this time yesterday?    

—I _____ basketball with my friends.     [ ]

A. was playing

B. had played

C. have played

D. played