In Drosophila,singed Bristles (Sn)and Cut Wings (Ct)are Both Caused by Recessive,X-Linked

Multiple Choice

In Drosophila,singed bristles (sn) and cut wings (ct) are both caused by recessive,X-linked alleles.The wild type alleles (sn⁺ and ct⁺) are responsible for straight bristles and intact wings,respectively.A female homozygous for sn and ct⁺ is crossed to a sn⁺ct male.The F₁ flies are interbred.The F₂ males are distributed as follows: $$\begin{array} { | l | l | } \hline s n c t & 13 \\\hline s n c t ^ { + } & 36 \\\hline s n ^ { + } c t & 39 \\\hline s n ^ { + } c t ^ { + } & 12 \\\hline\end{array}$$
What is the map distance between sn and ct?

A) 12 m.u.
B) 13 m.u.
C) 25 m.u.
D) 50 m.u.
E) 75 m.u.