13. Defense Department analysts worry that the ability of the United States to wage a prolonged war would be seriously endangered if the machine-tool manufacturing base shrinks further. Before the Defense Department publicly connected this security issue with the import quota issue, however, the machine-tool industry raised the national security issue in its petition for import quotas. Which of the following, if true, contributes most to an explanation of the machine-tool industry’s raising the issue above regarding national security? A. When the aircraft industries retooled, they provided a large amount of work for too builders. B. The Defense Department is only marginally concerned with the effects of foreign competition on the machine-tool industry. C. The machine-tool industry encountered difficulty in obtaining governmental protection against imports on grounds other than defense.

选Ｃ怎么解释啊，谢谢，呵呵，