单项选择题选出与该句匹配的段落：Rather than superpower conflict, countries unable to cope with food shortages now constitute the main threat to world security.?
单项选择题只承受弯矩而不承受扭矩的轴为( C )；既承受弯矩又承受扭矩的轴为（ D）；主要承受扭矩的轴为（ ）。
