If the range of the function f(x)=ax2+cf(x) = ax^2 + cf(x)=ax2+c is [2,∞)[2, \infty)[2,∞), which of the following must be true?
a>0a > 0a>0 and c=2c = 2c=2
a<0a < 0a<0 and c=2c = 2c=2
a>0a > 0a>0 and c=0c = 0c=0
a=1a = 1a=1 and c=−2c = -2c=−2