_____we are not attacked by nuclear weapons, we promise that under no circumstances _____ use the

bomb first but we reserve the right to use it for counter-attack when necessary.

[      ] 

A. As long as; we shall

B. If; shall we

C. So long as ; shall we

D. Even if; we shall

电杆在架空杆路中的地位来分为中间杆、角杆和终端杆三种。