What happens in the process of variation?

Which of the following statements are correct?

What is a linkage group?

Select the correct statement regarding dihybrid crosses and linkage.

The number of linkage groups in $E. coli$ is:

In a fruit fly,the $X$-chromosome linkage map has a total length of $66$ units,with the yellow body gene $(y)$ at one end and the bobbed hair gene $(b)$ at the other end. What will be the recombination frequency between these two genes $(y$ and $b)$?