太阳是太阳系里唯一的____。

    One day, Allan and his friend Henry went swimming in a river. It was very hot. How happy they were in the river! After they got out of the water, they played games in the sun for a while.
    On their way back, Henry saw some flowers. He liked flowers very much and ran into the green field to look at them. Now Allan was walking by himself.
    Then he heard Henry calling out, "A snake（蛇）! Help!..."
    "What's wrong with you?" asked Allan.
    "A snake bit（咬）me in the leg. Come here!"
    Allan ran over and saw a small red wound（伤口）on Henry' s leg.
    "The snake was in the grass. I didn't see it."
    "Sit down quickly"! Allan told Henry.
    Allan put his mouth at the little red wound and began to suck（吸）at it. In this way he saved （挽救）Henry's life. "Oh, Allan, it's very kind of you to help me."
    "That's all right. We are friends and we must always help each other."

中东所产石油主要输往（  ）

下列式子：①$\sqrt{\frac{1}{3}}$；②$\sqrt{-3}$；③﹣$\sqrt{x^2+1}$；④$\sqrt[3]{27}$；⑤$\sqrt{{\left(-2\right)}^2}$，是二次根式的有（　　）