A、By the end of this century. B、By the first half of this century. C、By the second half of this century. D、In the middle of this

With the development of science, scientists have discovered more about the world. Scientists say world coastal areas must prepare now for a rise in sea levels. No matter what action is taken, scientists say there are already enough greenhouse gases in the atmosphere to cause a major warning. As a result, scientists and government officials are focusing on how society might adapt to rising sea levels, changing agricultural zones, changing climates and other problems.
    Already, state and local governments along the coast of the United States are starting to plan for a possible longterm rise in the sea level, and a few have taken action to cope with it. Agricultural researchers are developing heat-resistant and draught-resistant crops that might replace traditional crops if the climate changes. How long it will take for maximum warming to be reached is uncertain because the oceans slow such warming. Most scientists expect the maximum warming to take place by the second half of this century.
    If levels of greenhouse gases in the atmosphere double, sea levels will rise; estimates of the average rise are from 1.5 feet to 6.5 feet. Some leading experts expect a rise of I meter, a little more than three feet.