When a scatter chart of data shows a nonlinear relationship, the nonlinear model can be expressed as
a. y= β₀ + β₁ x₁ + β₂ x₁² + ε
b. y= β₀ + β₁ x₁² + β₂ x₁² + ε
c. y= β₀ + β₁ x₁ + (β₂ x₁)² + ε
d. y= β₀ + β₁ x₁ + β₂ x₁²