小题1:B
小题1:D
小题1:A
小题1:D
小题1:C
小题1:根据In 1620, about half the USA was covered by forests．可知选B
小题1:根据Today the forests have almost gone． A lot of good land has gone with them, leaving only sand．可知选D
小题1:根据The Great Green Wall is 7,000 kilometres long, 可知选A
小题1:根据Trees must be grown all over the world．可知选D
小题1:根据Great Green Walls will make the world better．可知选C