A Testcross of Trihybrid Drosophila Produced the Following Phenotypes and Number

Multiple Choice

A testcross of trihybrid Drosophila produced the following phenotypes and number of offspring. A table showing phenotype and the number of offspring with each phenotype is below. A plus sign is used for wild-type phenotype; a letter indicates the mutant phenotype for that gene. $\begin{array} { l r } + + + & 669 \\a b c & 653 \\+ + c & 121 \\a b + & 139 \\+ b + & 2280 \\a + c & 2215 \\+ b c & 3 \\a + + & 2\end{array}$
-Which gene pair is closer together; i.e.there are fewer map units between them?

A) a ? b
B) b ? c
C) a ? c
D) a ? b and b ? c are the same length
E) a ? c and b ? c are the same length