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