(Multiselect) In an interrupted-conjugation experiment in E. coli, the pro gene enters after the thi gene. A pro(+) thi(+) Hfr is crossed with a pro(-) thi(-) F- strain, and exconjugants are plated on medium containing thiamine but no proline. A total of 360 colonies are observed, and they are isolated and cultured on fully supplemented medium. These cultures are then tested for their ability to grow on medium containing no proline or thiamine (minimal medium), and 320 of the cultures are found to be able to grow but the remainder cannot. Which crossover events are required to produce pro(+) thi(-)?