题目下列函数为有界函数的是.A. y = a^xB. y = arctan xC. y = x^2D. y = tan x下列函数为有界函数的是.A. $y = a^x$B. $y = \arctan x$C. $y = x^2$D. $y = \tan x$题目解答答案B. $y = \arctan x$