下列句子有语病的一项是（    ）

已知线段a是线段b、c的比例中项，b=3cm，c=12cm，则a=____cm．

泥石流的成因：

完形填空
      This is my desk, and 1 books are on it. One is yellow and the other(另外的) is blue. You can see a 2 on the desk, too. The clock is red. My pencil case is3 the desk, too. A  pen, an eraser  and a ruler4in it. The pen is black. The5 is yellow, and the ruler is white.
    6 is my pencil? Well, it's under the desk. A baseball is under the7too. But the baseball isn't 8. It's my brother's. Is my computer under the desk, too? No，9isn't. Where is it? I can't10it in the room.

如图，已知正方形ABCD的边长是4，点E是AB边上一动点，连接CE，过点B作BG⊥CE于点G，点P是AB边上另一动点，则PD+PG的最小值为____．