1. The underlined phrase that needs correcting is **A**: "do". The correct form should be “What do people do when there are too many people on Earth?”

2. The underlined word that needs correcting is **C**: "reach." The correct form should be “The world’s population will reach 11 billion by 2100.”

3. The underlined word that needs correcting is **A**: "terrifying." The correct form should be “I was terrified when I saw a UFO that has the shape of a disk.”