A:动能增大,重力势能减小,机械能不变
B:动能减小,重力势能增大,机械能不变
C:动能不变,重力势能减小,机械能不变
D:动能不变,重力势能减小,机械能减小
This chemical class is about the ______ of metals with oxygen and our teacher shows us how iron ______ air and water.