Use the sentences in the box to complete the conversation.

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
   First, deforestation leads to more CO2 in the air      I think it’s cutting down or burning forests      This also makes the earth even warmer      Well, there are two main reasons   
Mark: Mai, which human activity is the main cause of global warming?
Mai: .
Mark: Why do you think so?
Mai: . . Trees use CO2 to grow, so when they are cut down or burnt, CO2 in the air increases.
Mark: I see.
Mai: . Forests store carbon in the trees and soil, and help reduce the global temperature. That’s why I think cutting down or burning forests is the most serious problem.