单项选择题
A. VT circuits
B. STS circuits
C. VT tunnel circuits
D. 1:1 protection circuits
E. 1:N protection circuits
单项选择题 Click the Exhibit button. Given the information shown in the exhibit, when creating 2 DS1 circuits, how many STS ports are used on the VT1.5 matrix at Node 1 as VT circuits and how many are used after using VT tunnels?（）
单项选择题 A customer wants to create a 1:1 protection group with a DS3N-12 card. Which card slots would allow for a 1:1 protection?（）
单项选择题 Click the Exhibit button. Which type of protection is shown in the exhibit?（）