48.下列哪個函數的定義域恰為所有「正實數」？ 
(A)\(f_1(x) = \frac{1}{3} \log_{10}(2x)\)
(B)f2 (x)＝2sin(x − π ) 
(C)\(f_3(x) = \frac{2}{x}\)
(D)f4 (x)＝√5x

答案：登入後查看
統計： A(1), B(0), C(0), D(0), E(0) #3961479