—When were you born?—I was born ________ April 23, 1979.
下列条件中，容易形成降水的是（ ）①从低纬度流向高纬度的气流 ②从高纬度流向低纬度的气流③从高空中下沉到地面的气流 ④从地面上升到高空中的气流．
