①S_△ODB=S_△OCA；

②四边形OAMB的面积为2﹣a；

③当a=1时，点A是MC的中点；

④若S_{四边形OAMB}=S_△ODB+S_△OCA，则四边形OCMD为正方形．

其中正确的是____．（把所有正确结论的序号都填在横线上）

干____活（gān   gàn）    咬断____（dàn  duàn）   棉____花（mián  ｍín）

（已  以）____经        （非  飞）____常         （在  再）____见

（坐  座）____位         大____（像  象）        （午  牛）____后

    I have been to the Country Park many times. I think the Country Park is a b____ place for people to v____. There is a big l____ in the Country Park. We can go boating there. We always have a good t____ in the park. There are a lot of nice b____ in the park. They fly in the park and s____ in the park. How beautifully they can sing.