— Is Jack on duty today?

— It ____ be him; it's his turn tomorrow. [ ]

A. mustn't

B. won't

C. can't

D. needn't

下列服务组件中，______可以使用户在Linux与Windows操作系统之间实现文件系统和打印机共享功能。

A．FTP

B．Samba

C．DHCP

D．Squid