下列有关细胞学说的叙述，正确的是（　　）

抛物线y=x²+2x+3的对称轴是（　　）

    One sunny Sunday morning, Alex and Emma were playing football in the park. Their dad was sitting on a chair, reading a newspaper.

    Then Emma stopped and walked to Alex. “It's too hot,” she told him. “I think I need a sun hat.” “Me too!” Alex agreed.

    Emma looked at her dad's newspaper and said, “I have an idea.” She asked Dad for two sheets of newspaper and made each of them into a hat. Then she put one on Alex's head and the other on hers. “The hat is great!” Alex said.

    They sat down next to Dad and watched everyone in the park. Then the wind began to blow. Some people started flying kites. “I wish I had a kite,” Alex said. “Or a plane!” Emma said. She took off their hats and made them into two planes.

    They played with their planes until they heard thunder (雷声). “Oh, it's time to go home now,” Emma said. “Or all of us will get wet even if I make each of us a paper umbrella!”