根据提示用适当的介词完成句子。

1.—Where are my books?

    —They're o________ the sofa.

2.—Where is your pen?

   —It's i________ the pencil case.

3.—Where are my shoes?

   —They're u________ the bed.

4.—Where is my cat?

    —It's b________ the door.

下图是我国东部某沿海等高线分布图，据此完成下列要求。

a、b两条支流水流较急的是（），原因是（）。