What is the reason for the absence of hybrids?

$A$ test cross of an $F_1$ fly $+a/+b$ produces the following offspring:
$++/ab = 9$
$ab/ab = 9$
$+b/ab = 41$
$a+/ab = 41$
What is the distance between the linked genes?

If interference is complete or $100\%$ then the frequency of observed double crossover will be:

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$:
Mendel's law of independent assortment does not hold good for the genes that are located closely on the same chromosome.
Reason $(R)$:
Closely located genes assort independently.
In the light of the above statements,choose the correct answer from the options given below:

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:

Which method can be used to determine the location of specific genes on a chromosome?