7∶4和 $\frac{1}{7}$ ∶ $\frac{1}{4}$ 可以组成比例。

豌豆种子的黄色对绿色为显性，圆粒对皱粒为显性．现有黄色圆粒和绿色皱粒两个纯种品系，用它们作亲本进行杂交，再使F₁自交，得F₂，为获得稳定遗传的绿色圆粒豌豆，对F₂还应（　　）

阅读理解
Once there lived a very clever man whose name was Aesop (伊索). One day as he was taking a walk, he came across a traveler. The traveler stopped him and asked, “Excuse me,, can you tell me how soon I’ll get to town?”
“Go.” Aesop answered.
“I know I must go.” said the traveler, “but I want to know how soon I’ll get to town.”
“Go! ”Aesop said again angrily.
“This man must be mad(疯的),” the traveler thought and went on.
After the traveler had gone some distance(一段距离), Aesop shouted after him, “You’ll get to town in two hours.”
The traveler was greatly surprised. He turned back and asked, “Why didn’t you tell me that before?”
“How could I tell you that before?” Aesop answered, “I didn’t know how fast you could walk.”

我国的____ 岛位于台湾岛的东北方向，自古就是中国领土．