0eb517…: “As well as a careful fact”

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Consider the following contour plot for $z=F(x,y)$ :

Assume $F(x,y)$ either increases or decreases between contour lines. Let $\vec {p}(t)=\vector {x(t),y(t)}$ be a smooth vector-valued function that plots the curve above as $t$ runs from $a$ to $b$ . An arrow represents the direction that the curve is drawn. Select all points (A, B, C, D) where $\dd {t} F(\vec {p}(t))<0$ .

A B C D