设D的质量为X,根据质量守恒,列等式:50g+12g=30g+X,解得X=32g;设A的相对原子质量为Y,2A+B=C+3D50g 32g2Y 3×32可得比例式: 50g 32g = 2Y 3×32 ,解得Y=75.故选C