[单选题]
Substance A has a denh5c,tw bjy-*jsity yd. vu,6r;mit4fx-z l rvpu6 1of $3.0 \; g/cm^{3}$ and substance B has a density of $4.0 \; g/cm^{3}$. In order to obtain equal masses of these two substances, the ratio of the volume of A to the volume of B will be equal to
