Firewall is a （）mechanism used by organizations to protect their LANs from the Internet.

A．reliable

B．stable

C．peaceful

D．security

细胞膜在静息情况下，对下列哪种离子通透性最大 ( )

A．K⁺

B．Na⁺

C．Cl^-

D．Ca²⁺

E．Mg²⁺