Consider the following steps:

${\mathrm{Cu}}_2\mathrm{S}\stackrel{\text{roast in air}}{\to }\mathrm{A}\stackrel{\text{Heating without air}}{\to }\mathrm{B}$

Which is not the correct statement?

A is a mixture of ${\mathrm{Cu}}_2\mathrm{O}$ and ${\mathrm{Cu}}_2\mathrm{S}$ and B is a mixture of Cu and ${\mathrm{SO}}_2$

it is self-reduction

all statements are incorrect

A is only ${\mathrm{Cu}}_2\mathrm{O}$ and B is a mixture of Cu and ${\mathrm{SO}}_2$