What is the minimum percentage increase in the mean of set X {-4, -1, 0, 6, 9} if its two smallest elements are replaced

[BTGModeratorVI]

What is the minimum percentage increase in the mean of set X {-4, -1, 0, 6, 9} if its two smallest elements are replaced with two different primes?
(A) 25%
(B) 50%
(C) 75%
(D) 100%
(E) 200%

Answer: D
Source: Manhattan prep