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.

结构化程序设计以程序易读、易理解为目的，它是用三个基本控制结构： ______ 、选择、循环来编制程序。