Why Is Rate = K[HgCl₂] ²[C₂O₄^2-] Not the Rate Law

Why is rate = k[HgCl₂] ²[C₂O₄^2-] not the rate law for the following mechanism?
2 HgCl₂ + C₂O₄^2- → 2 Cl^- + 2 CO₂ + Hg₂Cl₂ (overall reaction)
HgCl₂ + C₂O₄^2- ⇌ HgCl₂C₂O₄^2- (fast)
HgCl₂C₂O₄^2- + C₂O₄^2- → Hg + 2 C₂O₄Cl^2- (slow)
Hg + HgCl₂ → Hg₂Cl₂ (fast)
2 C₂O₄Cl^2- → C₂O₄^2- + 2 Cl^- + 2 CO₂ (fast)

A) The rate law is not based on the slow step of the proposed mechanism.
B) The steps do not add to the overall reaction.
C) The rate law does not agree with the overall reaction.
D) The exponents of HgCl₂ and C₂O₄^2- are not equal.
E) The first step is not the slow step.