克隆羊的实例说明，____控制着生物的发育和遗传．

A

    Sue and Ann study in Beijing. This summer holiday they will go on a trip to Tianjin.

    Tianjin is about 137 kilometers from Beijing. How will they go there? By train, by bus or by bicycle? If they take a bus, the ticket is 35 yuan. If they take a train, the prices of the train tickets may be different because they can take different trains. The train tickets may be 11 yuan, 19 yuan, 22 yuan, 36 yuan or 58 yuan. Some are faster than buses. Some are slower than buses. They can take a train or a bus there, but they want to do something different. They want to go to Tianjin on their bicycles. On the way, if they are tired, they can stop to have a rest. They can also take pictures when they see beautiful places.

    They will leave early on the morning of July 8. They are very excited. They can arrive in Tianjin before 5 o'clock in the afternoon.

（注：在解题过程中，你也可以阅读后面的材料）

附：阅读材料

   任何一个一元二次方程的根与系数的关系为：两根的和等于一次项系数与二次项系数的比的相反数，两根的积等于常数项与二次项系数的比．

   即：设一元二次方程ax²+bx+c=0的两根为x₁，x₂，

   则：x₁+x₂=﹣ $\frac{b}{a}$ ，x₁•x₂= $\frac{c}{a}$

   能灵活运用这种关系，有时可以使解题更为简单．

   例：不解方程，求方程x²﹣3x=15两根的和与积．

   解：原方程变为：x²﹣3x﹣15=0

∵一元二次方程的根与系数有关系：x₁+x₂=﹣ $\frac{b}{a}$ ，x₁•x₂= $\frac{c}{a}$

∴原方程两根之和=﹣ $\frac{-3}{1}$ =3，两根之积= $\frac{-15}{1}$ =﹣15．