A.6x-4×0.9=0
        6x-3.6=0
    6x+3.6-3.6=0+3.6
            6x=3.6
         6x÷6=3.6÷6
             x=0.6
B.3×8-4x=0
    24-4x=0
 24+4x-4x=0+4x
       4x=24
    4x÷4=24÷4
        x=6
 C.4x=1.2-2x
4x+2x=1.2+2x-2x
   6x=1.2
6x÷6=1.2÷6
    x=0.2
故选：A．