It was quite a job for her to _____ several children all by herself during the war.[ ]

A. dress up    

B. bring up

C. break up

D. add up

隧道技术是VPN的基本技术，隧道是由隧道协议形成的，常见隧道协议有IPSec、PPTP和L2TP，其中 (1) 和 (2) 属于第二层隧道协议， (3) 属于第三层隧道协议。