Which agreement rule applies to a subject joined by or or nor?

a.) use a plural verb
b.) use a singular verb
c.) make the verb agree with the subject closest to the verb
d.) make the verb agree with the first subject listed

the triple alliance. germany and austria-hungary were already allies by 1879 when, worried about a threat from russia, they formed the dual alliance. italy joined the group in 1882. there was a concern about a possible attack from france and the alliance needed powerful friends to defend it.

the answer should be a. the sui empire, the tang empire extended from korea in the west to manchuria in the east.