[질문] The order of four genes in an Hfr strain is a b c d. In a cross between an Hfr donor that has genotype a+b+c+d+x- str-s and female that has genotype a-b-c-d-x+ str-r, 90% of the d+ str-r recombinations are x-, and 100% of the c+d+ str-r recombinants are x-. The times of entry of a,b,c and d are 5,10,15,20 minutes; the str gene enters at 55 minutes. Where is x located?