What is the result of linkage and crossing over?

The term 'crossing over' was coined by

$A$: Morgan's cross $III$ was conducted in $Drosophila$ to locate genes on the chromosome for white eye color.
$R$: The cross was done between red-eyed hybrid female and white-eyed male.

The map distance between genes $A$ and $B$ is $3$ units,between $B$ and $C$ $10$ units,and between $C$ and $A$ $7$ units. The order of the genes in a linkage map constructed on the above data would be:

What is the reason for the absence of hybrids?

Linkage in $Drosophila$ was discovered by: