Which of the following expressions is equal to ∑²⁰ₖ₌₅3ᵏ ?
a. 3⁵∑¹⁵ₖ₌₀3ᵏ
b. 3⁵∑²⁰ ₖ₌₅ 3ᵏ⁻⁵
c. 3⁻⁵∑¹⁵ₖ₌₀3ᵏ
d. 3⁵∑¹⁵ₖ₌₀ 3ᵏ⁺⁵k