题目

My brother wants to leave the company as it is ________ I would like to expand it. A . unless B . since C . before D . while 答案：D15．图为某高等哺乳动物雌性个体的一个细胞示意图，该细胞是（　　）A．卵原细胞B．初级卵母细胞C．次级卵母细胞D．卵细胞