As students, _______, we should study hard to be well prepared for the future.

 —Would you like to go shopping with me?       ———Yes,            ．  

We went to      earth.

如图，在平行四边形ABCD中，对角线AC,BD相交于点 O，若BD，AC的和为18 cm，CD：DA=2：3，△AOB的周长为13cm，那么BC的长是（      ）