1. Longest rail tunnel in the world ?

Answer: Seikan Tunnel (Japan - 53.9 Km)

2. Which is the lightest gas ?

Answer: Hydrogen

3. Which is the largest Zoo in India ?

Answer: Zoological Gardens, Alipur, Kolkata

4. Which is longest river of India

Answer: Ganges

5. Which is Longest road tunnel in the World ?

Answer: Lærdal Tunnel (Norway - 24.51 kilometre)

6. Which is the shortest day in the year ?

Answer: December 22

7. Which is highest dam in world ?

Answer:

Jinping-I Dam - 305 m (1,001 ft) in China on Yalong river.
Second Highest is Nurek Dam 300 m (980 ft) Tajikistan on Vakhsh river.