1£®As a young doctor, John Snow became frustrated because no one knew how to prevent or treat cholera.

2£®One theory was that bad air caused the disease.

3£®The other was that cholera was caused by an infection from germs in food or water.

4£®The truth was that the water from the Broad Street pump had been infected by waste.

5£®That's why we've given you the letter.

6£®It looks as if it is going to rain.