If A, B and C be the angles of a triangle, then which of the folowing  incorrect relation is

A

$$sin \frac{A+B}{2}= cos \frac{c}{2}$$

B

$$cos \frac{A+B}{2}= sin \frac{c}{2}$$

C

$$tan \frac{A+B}{2}= sec \frac{c}{2}$$

D

$$cot \frac{A+B}{2}= tan \frac{c}{2}$$