Correct a mistake

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IV. Correct a mistake
1. Saturn is the second largest planet after Jupiter, with a diameter nearly ten times those of Earth.
2. Competitive cyclists must react calm during tournaments.
3. According to my opinion, the pollution of the air by factory smoke is one of the major problems.
4. I shall never win John at tennis; we are clearly not in the same class.
5. Knowing how to use words perfectly are important in learning a foreign language.
6. This matter is very confident, so don't discuss it outside this office.
7. Sometimes advertising signs blow down during a storm and people are injured by them.
8. Over the past 50 years many serious illnesses have been brought in control.
9. The six-years-old boy always goes to church with his parents on Sundays.
10. The doctor advised him to avoid eating fatty foods, having more fresh fruit and drink much water.