David was a little boy. He1 four years old. One day his mother  2him to see his grandmother. His grandmother's home was very far 3 them, so they went there4 . There 5 only a few(一些) people on the bus. They all sat on  6 seats. David was 7 . He looked out of the window and talked with his mother.

    A moment later (一会儿以后), David started running on the bus. He ran and ran(不停地跑来跑去). "8 are you always running? Please sit9 ." His mother said. "I want to see my grandmother 10 . I want the bus to go faster," David answered.

人体有病菌侵入时白细胞数目会大量增加，这是因为白细胞具有（　　）

明朝之所以能出现郑和下西洋的壮举，主要原因是（ ）