A chromium(III) ion forms a complex ion with two ammonia molecules and four thiocyanate ions. What is the formula of the complex ion?

a. [Cr(NH₃)₂ (NCS)₄]⁴⁻
b. Cr(NH4)₂(NCS)₄]⁺
c. [Cr(NH₃)₂(NCS)₄]³⁺
d. [Cr(NH₃)₂(NCS)₄]⁻