Task: Question Answering
Input (English): Background Paragraph: Prokaryotes have a different way to increase genetic variation. It’s called genetic transfer or bacterial conjugation . It can occur in two ways. One way is when cells “grab” stray pieces of DNA from their environment. The other way is when cells directly exchange DNA (usually plasmids) with other cells. For example, as shown in Figure below , the donor cell makes a structure called an F pilus , or sex pilus. The F pilus attaches one cell to another cell. The membranes of the two cells merge and genetic material, usually a plasmid , moves into the recipient cell. Genetic transfer makes bacteria very useful in biotechnology. It can be used to create bacterial cells that carry new genes. 
Story: Two researches studied living organisms, Joe studied eukaryotes, while Bill studied prokaryotes. They were fascinated by genetic variation. 
Question: Which researcher observed more cells directly exchange DNA (usually plasmids) with other cells?
Output (English): Bill.