15÷3=____ 21÷5=____……____
28+45=____ 4×7=____
300+700=____ 418+200=____
640-600=____ 396-300=____
320+400=____ 53-35=____
About of the workers in the factory were born in the .
某学生为研究乙二酸(HOOC﹣COOH)的化学性质,进行如下实验:向盛有硫酸酸化的高锰酸钾溶液的试管中滴入适量乙二酸饱和溶液,振荡,观察到试管中溶液由紫红色变为无色.由此可知,乙二酸具有( )