某种种子每粒发芽的概率有都为0.9，现播种了1000粒，对于没有发芽的种子，每粒需再补种2粒，补种的种子数记为X，则X的数学期望为____．
The fire started on the second floor and it wasn't long ______ the whole building was burned down.
下列各图反映的地理现象，主要由外力作用形成的是（ ）
