My sister can ______.[ ]

A. sing

B. sings

C. to sing

D. singing

证明可微的必要条件定理：设z=f(x，y)在点(x₀，y₀)处可微，则f’_x(x₀，y₀)与f’_y(x₀，y₀)都存在，且

并请举例说明(2)之逆不成立．