The Reduction of M³⁺ by Cr²⁺ Was Studied with [Cr²⁺]

The reduction of M³⁺ by Cr²⁺ was studied with [Cr²⁺] 100 times the concentration of Cr²⁺.When [Cr²⁺] = 0.0050 M,the rate was 2.5 s^-1.The rate constant for this reaction is

A) 1.3 M^-1·s^-1
B) 0.013 M^-1·s^-1
C) 5.0 M^-1·s^-1
D) 500 M^-1·s^-1