A physicist is measuring the net magnetic flux density $B$ inside a custom solenoid as the control current $I$ is adjusted. The physicist correctly evaluates the flux density at two specific current levels:

$$B(-5)=-2andB(5)=2$$

and concludes that there must be an equilibrium current level in the interval $I\in (-5,5)$ where the flux density is exactly zero ($B=0$), citing the change of sign. For which of the following models for $B(I)$ is this conclusion incorrect?

• $B(I)=0.4I$
• $B(I)=\frac{10}{I}$
• $B(I)=0.016I^3$
• $B(I)=\frac{6I}{I+10}$