The first answer is C. The information may be in a form that is difficult to use. And the second on is D. They must always be used with an equation.
Carmen has given 6 coins to her brother<span />
If there is such a scalar function <em>f</em>, then



Integrate both sides of the first equation with respect to <em>x</em> :

Differentiate both sides with respect to <em>y</em> :


Integrate both sides with respect to <em>y</em> :

Plug this into the equation above with <em>f</em> , then differentiate both sides with respect to <em>z</em> :



Integrate both sides with respect to <em>z</em> :

So we end up with

Yes.
It’s a flat figure (2D), and all of its lines are connected with one another. There aren’t any open points/lines. It’s closed to create a shape. This means it’s a polygon.