Two distinct bands appear in a single lane at the completion of an SDS-PAGE analysis of an unknown protein. Select one: a. These bands represent two subunits of a homodimer. b. Each band represents a different sized subunit of a multimeric protein. c. Each band represents a protein subunit with different charge to mass ratio. d. These bands represent two subunits of a heterodimer. e. These bands represent two separate subunits of identical size.