4. The first step in two-dimensional gel electrophoresis generates a series of protein bands by isoelectric focusing. In a second step, a strip of this gel is turned 90 degrees, placed on another gel containing SDS, and an electric current is again applied. In this second step: (A) proteins with similar isoelectric points become further separated according to their molecular weights. (B)the individual bands become stained so that the isoelectric focus pattern can be visualized. (C) the individual bands become visualized by interacting with protein-specific antibodies in the second gel. (D)the individual bands undergo a second, more intense isoelectric focusing. (E)the proteins in the bands separate more completely because the second electric current is in the opposite polarity to the first current.