It is known that the function f (x) whose domain is r decreases monotonically in the interval (- ∞, 5). For any real number T, f (5 + T) = f (5-T), then the following formula must be true () A. f（-1）＜f（9）＜f（13）B. f（13）＜f（9）＜f（-1）C. f（9）＜f（-1）＜f（13）D. f（13）＜f（-1）＜f（9）

It is known that the function f (x) whose domain is r decreases monotonically in the interval (- ∞, 5). For any real number T, f (5 + T) = f (5-T), then the following formula must be true () A. f（-1）＜f（9）＜f（13）B. f（13）＜f（9）＜f（-1）C. f（9）＜f（-1）＜f（13）D. f（13）＜f（-1）＜f（9）

The image of ∵ f (5 + T) = f (5-T) ∵ function f (x) is symmetric with respect to x = 5 ∵ f (- 1) = f (11), ∵ function f (x) decreases monotonically in the interval (- ∞, 5), and ∵ function f (x) increases monotonically in the interval ((∞, 5), + ∞)