A cross is made between the streptomycin$-$resistant $($Str$)$ F$-$ strain marked with the characters Gal T ARLac T$]$R Mal Xyl Li and the prototrophic Hfr strain having the opposite characters. After $60$ minutes d contact, samples are transferred to plates with minimal medium plus streptomycin. All markers can be transferred within this time. Key to characters: T and L are amino acids; Gal, Lac, Mal, Xyl are sugars; Az$=$ azide; $71$ is phage; Str $=$ streptomycin. The original mixture is in the ratio of $2$ x $10\text{'}$ Hfr to $4$ x $108$ F$-.$The frequencies $($percentages$)$ of the fr characters recovered in the T$+$L$+$ SR recombinants are as follows: $72\%$ T$/,0\%$ Mal$+,27\%$ Gal$+,91\%$ AzS, $0\%$ Xyl$,48\%$ Lac$+.$ Note: The order of $($T and L$)$ and $($Mal and Xyl$)$ cannot be determined from these data. $(6$ pt$)67.($a$)$ What is the counterselective agent that prevents the Hfr individuals from obscuring the detection of recombinants?$68.($b$)$ In what order are these genes probably being transmitted by the Hfr$?($T$+$L$+$ must come in first, and they must be closely linked.$)69.($c$)$ What is the meaning of $0\%$ Mal$+$ and $0\%$ Xyl$+?$ Where do you think these markers are located on the chromosome relative to StrR$?$