Though there are some differences in American education and Chinese education, both systems are _____

excellent graduates.

[      ]

A. turning out

B. turning away

C. turning off

D. turning in

果蝇的红眼和白眼是由X染色体上的一对等位基因控制的一对相对性状。一对红眼雌、雄果蝇交配，子一代中出现白眼果蝇。让子一代果蝇自由交配，理论上子二代果蝇中红眼与白眼的比例为（）。

A.3∶1

B.5∶3

C.13∶3

D.7∶1