题目

—Have we _______ food? —Yes, _______．We’d better buy some． A．run out of; we have run it out                      B．run out of; our food has run out C．run out; our food has been run out              D．run out; our food has run out 答案：B皮肤能够阻挡病原体侵入人体，这属于人体防御病原体的（ ）A．第一道防线 B．第二道防线 C．特异性免疫 D．人工免疫