51. What is a tornado?
52. Which country do tornadoes often hit according to the passage?
53. How many levels of tornadoes are there?
54. What are the wind speeds of strong tornadoes?
55. What can you do when there is a violent tornado?
A spinning tower of air that goes from the sky to the ground.
52. Which country do tornadoes often hit according to the passage?
The United States.
53. How many levels of tornadoes are there?
Three.
54. What are the wind speeds of strong tornadoes?
Between 178 km/h and 322 km/h.
55. What can you do when there is a violent tornado?
We can keep calm and stay at home.
答案：51.A spinning tower of air that goes from the sky to the ground. 52.The United States. 53.Three. 54.Between 178 km/h and 322 km/h. 55.We can keep calm and stay at home. (言之有理即可)