Answer the following question given this paragraph: 

Lightning is the discharge of static electricity that has built up on clouds. Every year, the earth experiences an average of 25 million lightning strikes. Lightning bolts travel at speeds up to 60,000 miles per second, and can reach temperatures of 50,000°F, which is five times the temperature of the surface of the sun. The energy contained in a single lightning strike could light a 100 Watt light bulb 24 hours per day for 90 days.


Q: Where do lightnings build up?

 Choices:

- stratosphere

- air

- troposphere

- clouds

A:
clouds
