In a strain of fruit fly, after several times of artificial hybridizations, the map distances of the given loci is A-B=10 m.u., A-C=20 m.u., A-D=25 m.u., B-C=30 m.u., B-D=15 m.u., If the genetic map is drawn according to these humbers, the correct locus order should be ______.

A．X chromosome linked genes are easier than somatic genes
B．X chromosome linked genes are more difficult than somatic genes
C．X chromosome linked genes are as easy as somatic genes
D．All the above statements are incorrect because the data of pedigree method is unable to be used in chromosome mapping

A．equal division during sperm generation
B．abnormal mitosis during merogenesis (fertilized egg cleavage)
C．meiosis in embryo
D．meiosis when sperm is tailed