`1,` C

- Dẫn chứng (2 câu đầu) : Every four to ten years something happens to the weather system of the Earth. Everything changes, and scientists don't know why.

`2,` D

- Dẫn chứng (đầu đoạn 2) : Usually the effects of EL Nino are very mild. However, the one that started in 1982 was different from the other recent ones. It had a very powerful effect on the world's climate.

`3,` C

- Dẫn chứng : However, the one that started in 1982 was different from the other recent ones. It had a very powerful effect on the world's climate.

`4,` A

- Dẫn chứng (cuối đoạn 1) : It usually happens at Christmas time, and EL Nino is the Spanish name for the baby Jesus.

`5,` C

- Dẫn chứng (đầu đoạn 3) : In 1983, the winds, ocean currents, and pressure areas started to become normal again. Meanwhile, scientists throughout the world organized research to learn more about EL Nino.

`1,`C. are still unknown to scientists nowadays. (DC : Every four to ten years something happens to the weather system of the Earth`.` Everything changes, and scientists don't know why.)

`2.`D. both A and C. (DC : Usually the effects of el Nino are very mild. However, the one that started in 1982 was different from the other recent ones. It had a very powerful effect on the world's climate.)

`3,`C. more powerful and destructive than the other recent ones.(DC : However , the one that started in 1982 was different from the other recent ones. It had a very powerful effect on the world's climate.

`4.`A. EL Nino usually happens at Christmas time.(DC :  It usually happens at Christmas time, and EL Nino is the Spanish name for the baby Jesus.)

`5,`C. have organized research to learn more about EL Nino(DC :  Meanwhile , scientists throughout the world organized research to learn more about EL Nino.)

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