Consider the Set of Graphs Shown in the Column to the Right

Consider the set of graphs shown in the column to the right. These graphs show plots of $\phi(x, y, z)$ versus $x$ for fixed $y$ and $z$ in various electric fields. In comparisons, treat negative values as being smaller than positive values.
-(b) In which case is the value of $E_{x}$ smallest at $x=2 \mathrm{~cm}$ ?