Final answer:
Lanes 3 and 4 contain a single band because they have the smallest DNA molecules. Lanes 2, 5, 6, 7, and 8 contain 2 bands because they have larger DNA molecules.
Step-by-step explanation:
In gel electrophoresis, DNA molecules are separated based on their size. The smaller DNA molecules will travel faster through the gel, while larger DNA molecules will travel slower. In this case, lanes 3 and 4 contain a single band because they have the smallest DNA molecules, which travel the fastest. Lanes 2, 5, 6, 7, and 8 contain 2 bands because they have larger DNA molecules, which travel slower and get separated into two distinct bands.