$a)\;\large\frac{d_1^2v_1}{{d_2}^2} \\ b) \; \large\frac{d_2v_1}{d_2} \\ c) \;\large\frac{d_1v_1}{d_2}\\ d) \large\frac{{d_2}^2 v_1}{{d_1}^2}$