a,
$M_A= 12x+ 2x+2= 14x+2$
$M_B= 12x'+ 2x'= 14x'$
$M_C= 12x'+ 2x' - 2= 14x' - 2$
b,
$d_{B/A}= 1,4$
=> $14x'= 1,4(14x+2)$
$\Leftrightarrow -19,6x+ 14x'= 2,8$ (1)
$d_{A/C}= 0,75$
=> $14x+2= 0,75(14x'-2)$
$\Leftrightarrow 14x - 10,5x'= -3,5$ (2)
(1)(2) =>$ x=2; x'=3$
Vậy A là $C_2H_6$, B là $C_3H_6$, C là $C_3H_4$