Reason:

$(2)$ Consider the surface $in{R}^3$ given $byS=\{e^{f(y,z)}=g(x)\},$ where $f,g$ are both $C^1$ and

$d\frac{g}{d}x$ never vanishes. $Itis$ impossible for $Sto$ have any tangent planes parallel $to$ the

$yz-$plane $if$:

$(A)f(y,z)=y+z$

$(B)f(y,z)=z^3$

$(C)f(y,z)=y^2+z^2$

$(D)$ Two $of$ the above

$(E)$ All $of(A)-(C)$

Reason: