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.
D. A few weapons important for defense consist of parts that do not require extensive machining.
E. Several federal government programs have been designed which will enable domestic machine-tool manufacturing firms to compete successfully with foreign toolmakers.
做完此题之后，有一点困惑，请问import quotas是什么东东？麻烦各位大侠来帮忙扫一下盲－－给一点专业的基础知识。谢谢。 | 
