Assume that you are able to examine a total of 100 separate meiotic events in an animal species. You note that within 10 of these meiotic events, there was a crossover event occurring between genes A and
B: In the remaining 90 events there was no crossover event between these two genes. What would be the expected map distance between genes A and B?
选项：

A:10 m.u.;
B:5 m.u.;
C:20 m.u.;
D:45 m.u.;
E:50 m.u.