有一块正方形手帕，它的一条边长20厘米，要准备在这块手帕上镶一条漂亮的花边，应该准备多长的花边？
20×4=80（厘米）．
答：应该准备80厘米长的花边．
成熟B淋巴细胞的表面标志是（）
A.CD2
B.CD3
C.CD19
D.CD80
E.CD4
Green economy offers a new model ________ people could both protect the climate and develop the economy, according to what Li said at the International Cooperative Conference on Green Economy and Climate Change.
A．how
B．why
C．where
D．which
