4．TUVALU，a tiny country in the Pacific Ocean，has asked for help as it fears it will be swallowed up by the sea．
Storms and huge waves are a constant threat and none of Tuvalu's nine little islands is more than five meters above sea level，Salt water is already entering the country's drinking water supply，as well as damaging plants that produce fruit and vegetable． （36）C
（37）D． Venice，a historic city in Italy best known for its canals，has sunk about 24cm over the past 100years． Experts say that it will have sunk another 20-50cm by 2050． A century ago，St． Mark's Square，the lowest point in the city，flooded about nine times a year，Nowadays，it happens more than 100times． While Venice is slowly sinking into the mud on which it stands，Tuvalu's rising sea level is caused by global warming．
The average global temperature has increased by almost 0.5centigrade degrees over the past century； scientists expect it to rise by extra 1-3degrees over the next 100years．
Warmer weather makes glaciers（冰川）melt，adding more water to the ocean． The warmer temperatures also make water expand，so it takes up more space，causing the sea level to rise． The sea level has risen about 10-25cm in the last 100years．
（38）A． Through burning coal，oil and gas，people have been increasing the greenhouse gases in the atmosphere，such as CO2． This adds to the power of the greenhouse effect，making the planet even warmer．
Many scientists believe that，if the warming is not stopped，there will be huge climate changes．（39）E．
（40）B，the sea will swallow up millions of homes and the world will be flooded with"climate refugees"looking for somewhere to live．
A． The main cause of global warming is human pollution
B． Should this come true
C． Without urgent help，the country's days are numbered．
D． But Tuvalu is not the first place to face sinking into the sea
E． The sea level could rise by one meter this century
F． people face a lot of pollution
G． much has been done about our planet．
