Which of the Following Is NOT an Explanation for the Expansion

Which of the following is NOT an explanation for the expansion of Viking forces at the end of the first millennium C.E.?

A) A period of climatic cooling forced Vikings to seek out new land to inhabit.
B) The rise of centralized states may have forced Viking rebels to seek their fortunes elsewhere.
C) Vikings were adept sailors and had traded frequently with Romans.
D) A period of climatic warming caused Viking populations to rise, resulting in the need for new sources of supplies to sustain the larger number of people.