Rate expression for ammonia synthesis is___________________?

Rate expression for ammonia synthesis is___________________?

A. Kc=x2/(a-x)(b-x)
B. Kc=x2/v(a-x)
C. Kc=4×2/(a-2x)2(b-x)
D. Kc=4x2v2/(a-x)(b-3x)3