A. They think they are mad. B. They feel very tired. C. They become violently sick. D. They are too tired to do anything. 参考答案:B 答案解析:细节题 见第二段第一句When a large number of positive ions are present in the airmany people experience unpleas
第25题: [单项选择]聚焦声束与非聚焦声束描述中,错误的是()。 A. 聚焦区超声束会出现变细现象 B. 聚焦后其近场区声能分布变得均匀一致 C. 远场区非聚焦部分分散现象依然存在 D. 聚焦可减少远场声束扩散 E. 聚焦可改善图像的横向或侧向分辨力 参考答案:B 答案解析:采用聚焦技术,可使聚焦区超声束变细,减少远场声束扩散,改善图像的横向或侧向分辨力。聚焦后近场区(旁瓣区)声能分布不均现象依然存在。远场区的非聚焦部分散焦现象依然存在,某些单阵元探头或质量低劣的探头或许