含硫元素物质间发生转化:SSO2SO3H2SO4Al2(SO4)3,其中硫元素化合价发生变化的是( )
A:①②
B:③④
C:①③
D:②④
—So you gave Mary your dictionary?
— ______.She said she'd return it to me when she could afford her own.
下面四幅图中能说明电动机原理的是( )