The function f(x) = 25(4)x represents the growth of a bird population every year in a national park. Kyle wants to manipulate the formula to an equivalent form that calculates every half-year, not every year. Which function is correct for Kyle's purposes?
f(x) = 25(42)
f(x) = (4)x
f(x) = 25()2x
f(x) = 50(2)x