已知集合U = {(x，y)| xÎR, y ÎR}, M = {(x，y) | |x | + | y | < a }，P = {(x，y)| y =" f" (x ) }，现给出下列函数： ①y = a^x , ② y = log_ax , ③y =" sin(x" + a), ④y =" cos" a ^x，若0 < a < 1时，恒有P∩C_UM = P，则f (x)可以取的函数有

A．①②③

B．①②④

C．①③④

D．②③④