Let X₁, X₂, X denote a random sample of size ₃ from a gamma distribution with a = ₇ and θ = ₅. What is the distribution of the sum X₁ + X₂ + X?
a) Normal distribution
b) Exponential distribution
c) Gamma distribution with a = ₂₁ and θ = ₅
d) Poisson distribution
