In which of the following can recombination be observed?

Drosophila has four pairs of chromosomes. How many linkage groups does it have?

Genes $A, B$,and $C$ are linked. Genes $A$ and $B$ are closer than $A$ and $C$. Find out the correct option for the given statements:
$I.$ $A$ might be before $B$ and $C$
$II.$ $B$ might be between $A$ and $C$
$III.$ $C$ might be between $A$ and $B$
$IV.$ More crossovers have occurred between $A$ and $C$ than $A$ and $B$.

In a test cross involving $F_1$ dihybrid flies,more parental-type offspring were produced than the recombinant-type offspring. This indicates

Select the correct statement regarding dihybrid crosses and linkage.

The distance between two linked genes on a chromosome is measured in cross-over units,which is: