Check all the statements about an arbitrary function gs, 3;) and an arbitrary point (a, b) that are true: Cl f(a:, 3;) must have a tangent plane at (a, b). D If f(:1:, y) is differentiable at (a, b) then it is continuous at (a, b). 6 6 C] If 6): and (iif exist in a neighborhod of (a, b) and are continuous at (a, b) then ag y) is 3" y differentiable at (a, b). C] If f(:1:, y) is continuous at (a, b) then it is differentiable at (a, b). C] h'm f(:1:, 3;) always exists. (WWW611\" (3 6 Cl 6: and 6: always exist. C] If f(:1:, y) is differentiable at (a, b) then a tangent plane to f(:1:, :9) exists at (a, b)