f(x1) - f(x2)=ax1² + 2ax1 + 4 - ax2² - 2ax2 - 4=ax1² - ax2² + 2ax1 - 2ax2=a(x1 + x2)(x1 - x2) + 2a(x1 - x2)=a•0•(x1 - x2) + 2a(x1 - x2)=2a(x1 - x2)∵x10∴2a(x1 - x2)<0则f(x1) - f(x2)<0即：f(x1)∴选A
