Possible Duplicate:
[Singular] Is/Are [Plural]?
Singular or plural following a list
Verb agreement in “Where is the Messiah and his Kingdom?”
I am having trouble with the following sentence:
"The complexity and diversity of the new platforms is worrying us."
If "complexity" and "diversity" constitute two subjects, then the verb should presumably be "are"; however, the sentence feels more natural with "is". Is this correct? Is there a linguistic term for this occurrence?