设i , j为类X中定义的double型变量名，下列X类的构造方法中不正确的是( )

A. double X(double k ){ i=k; return i; }
B. X( ){i=6;j=8; }
C. X(double m, double n){ i=m; j=n; }
D. X(double k ){ i=k; }