What type of e$-$commerce involves online auction platforms where consumers bid for products or services?

$$a$.$B$2$C $($Business$-$to$-$Consumer$)$

$$b$.$C$2$C $($Consumer$-$to$-$Consumer$)$

c$.$B$2$B $($Business$-$to$-$Business$)$

d$.$C$2$B $($Consumer$-$to$-$Business$)$