[单项选择]Bystanders, ______, ______ as they walked past lines of ambulances. A. bloody and covered with dust, looking dazed B. bloodied and covered with dust, looked dazed C. bloody and covered with dust, looked dazed D. bloodied and covered with dust, looking dazed
第5题: [单项选择]超声波在两种介质之间会发生明显的反射回波，则这两种介质间的声阻差应为 A. ＞0.01% B. ＞0.1% C. ＞1% D. ＞10% E. ＞0.1‰ 参考答案:B 答案解析:[解析] 超声界面反射的特点是非常敏感。两种介质之间的声阻抗只要相差0.1%(声阻差)就会产生明显的反射回波。
