At their intersection point (x0,y0), two differentiable curves are said to share a common tangent. Which of the following conditions must hold at (x0,y0)?
a) Their second derivatives are equal at (x0,y0).
b) Their slopes are equal at (x0,y0).
c) The product of their slopes at (x0,y0) equals −1.
d) The magnitudes of their slopes are equal at (x0,y0).