Which of the following formula is not a multiple linear regression model when: Y = Salary, X₁ = Experience, X₂ = Age, X₂ = Level

a)Y = aX₁ + bX₂ + c

b)Y = aX₁ + bX₂ + cX₃

c)Y = aX₁ + bX₂ + cX₃+ d

d)Y = aX₁ + b(X₂)²

e)Y = aX₁ + bX₂