Let C be the curve defined by x2+y2=36. Consider all points (x, y) on curve C where y>0. Which of the following statements provides a justification for the concavity of the curve?

The curve is concave down because y′′=−2yx2<0.
The curve is concave up because y′′=2yx2>0.
The curve is concave down because y′′=−36y3<0.
The curve is concave up because y′′=36y3>0.