(74) systems perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and (75) on the disk, and controlling peripheral devices such as disk drives and printers.

A．contents

B．directories

C．directions

D．direct

胶合板的层数可以是双数，也可以是单数，可根据市场上的情况选购不同的品种.。