Suppose a solution of the differential equation $$(xy^3+x^2y^7)\frac{\mathrm{d} y}{\mathrm{d} x}=1,$$ satisfies the initial condition $y(1/4)=1$. Then the value of $\dfrac{\mathrm{d} y}{\mathrm{d} x}$ when $y=-1$ is
- $\frac{4}{3}$
- $- \frac{4}{3}$
- $\frac{16}{5}$
- $- \frac{16}{5}$