A:has not heard of
B:had not heard of
C:has not been heard of
D:had not been heard of
读甲、乙、丙、丁四幅图(如图),回答下题.
①是秦朝蜀郡守李冰主持修建
②分为鱼嘴、宝瓶口、飞沙堰三大主体工程
③是一座单一性的防洪工程
④建成后一直发挥巨大作用,充分体现了我国人民智慧
56+13 =____
=____
能证明乙酸是弱酸的实验事实是( )