Which of the following is not true for positive real numbers a, b? Why?

A: log((a)^x(b)^y) = x log a + y log b;

B: (a/b)^x = (a)^x(b)^x;

C: a^b = 10^(b log a);

D: log(a^x + b^x) = x log(a + b) ;

E: sqrt(a^x) = a^(x/2)